Why are weeds regarded as farmer's enemies?

Weeds suck-up a lot of water. So your plants won't get as much water as they need.

In the end they'll die

The main reason is that weeds rob the other plants of nutrients that they need to thrive. Example, (oxygen, phosphorus, potassium) and water. The root systems of some weeds are very invasive and take over an area quickly. They can be difficult and expensive to get rid of them. Secondly, from an aesthetic viewpoint the weeds begin to crowd out the nice plants that cost you money to place in the garden and it looks unkempt. A good way to prevent weeds is to mulch the planting areas with a covering of bark mulch (shredded redwood or cedar, small bark, or chips) about 2" thick. Replace as needed.