Why do people believe in evolution?

First of all, let's make it clear that there is a distinction between evolution the biological phenomenon on one side, and evolution the model explaining observations in terms of what we know about that phenomenon on the other. I propose that acceptance of an observed fact is hardly an interesting subject for a question and that the above question therefore deals exclusively with the acceptance of the theoretical model of evolution.

There can be any number of reasons for people to believe in evolutionary theory.

The first category of 'believers' is of course the one that contains the scientists in relevant fields of research, such as evolutionary biologists, palaeontologists, geneticists, molecular biologists, developmental biologists, and so on. They accept evolutionary theory as fundamentally accurate because it has been verified, proven to be a fundamentally accurate reflection of the available data. As the people in this group are intimately familiar with both the proposed theories and hypotheses as well as the observational data on which they are based, they have good reason to do so.

But the group of people accepting evolutionary theory because of their familiarity with the subject matter and supporting data is not at all limited to scientists. Many laymen have a more or less proficient understanding of the proposed models, as well as the relevant observational data, and, not unimportantly, scientific principles in general.

There is a much larger group of people that are remotely aware of the existence of the theory and some of the supporting observations, but not intimately enough to form an educated opinion for themselves. Many of these people accept evolutionary theory primarily on the basis that it's the prevailing (and indeed only) theory among experts in the relevant fields of science. This is a more or less valid reason to accept any scientific model, but it will be clear that such people cannot be expected to argue the correctness of their belief in other terms.

A group of people that is perhaps equally large simply don't ponder the question. Perhaps because they have more important matters to attend to (like surviving), perhaps because such matters are simply never brought under their attention, or perhaps because the subject simply doesn't interest them. While they cannot be said to actively reject the model, they can't be said to deny it either, which is why I will mention them here.

Finally, there will be a much smaller group of people who believe evolutionary theory is true, not because of knowledge of the subject, or trust in the scientific establishment, but for personal or ideological motivations, such as an inability to accept claims involving gods or aliens.

Another AnswerThere can be any number of reasons for people to believe in evolutionary theory. These are the ones that are the scientists in relevant fields of research, such as evolutionary biologists, palaeontologists, geneticists, molecular biologists, developmental biologists, and so on. They accept evolutionary theory as fundamentally accurate because it has been verified, proven to be a fundamentally accurate reflection of the available data. As the people in this group are intimately familiar with both the proposed theories and hypotheses as well as the observational data on which they are based, they have good reason to do so.
People believe in evolution because that is a fact of science, just as they believe in gravity and the Theory of Relativity.

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Charles Darwin, although not the first to recognise that species must have evolved over the ages, spent several decades of his life researching his Theory of Evolution by Natural Selection. When you have read The Origin of Species, that research is an impressive achievement.

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Others have continued the work of Darwin, over the ensuing 150 years. And the fossil record had added significantly to our understanding of evolution.

AnswerPeople believe in evolution because they believe it to be a fact of science. The fact that they frequently liken evolution to proven laws of science such as the Law of Gravity is certainly misleading if not downright deceptive in purely scientific terms. The Law of Gravity is both testable and repeatable in the present. It has no known exceptions or contradictions which is why it has the status of a 'Law' of science since it adequately describes proven scientific facts.

Evolution on the other hand cannot at all be likened to the law of gravity since it has not been proven. It also has many scientific facts against it. The fact that those who believe it frequently liken it or seek to associate it with the Law of Gravity indicates that they have more than just science in mind. They fervently believe in it as part of a naturalistic world view.

Some other people, possibly the majority, believe evolution because they have been taught it in school as a fact of science and are not aware of the whole picture which science has to offer. Nor are they aware of the many facts of science which are against it, nor of the deliberate frauds which have been put forward in favor of it.

Darwin himself saw problems with the many gaps in the fossil record in his day but believed they would be 'filled in' in time. However, despite much more research the missing links are still missing. The fossil record has indeed added significantly to the scientific understanding of the tenuous status of the theory of evolution. It does not support it. This adds further to the perception that belief in evolution is more than just following the scientific facts.

AnswerBecause among the many theories about the origin of man (and animals) it is only Evolution that is supported by tangible evidences (all others are mere hypothesis, conjectures at most wistful thinking) although not complete but are still evidences. Evolution is irresistibly here. What started as a snowball will eventually turn into an avalanche. AnswerBecause of the overwhelming evidence supporting evolutionary theory.