Why is it that the quality of services is more difficult for consumers to evaluate than the quality of products?

Products have a fairly set measurable. They are expected to work as advertised and it's fairly easy to see if it isn't. eg: A clock doesn't show the time.

Service is harder because there's different levels of expectation in both the provider and the consumer. There's also people's opinions, which are very subjective. One person may think that a checkout clerk that simply states the total is acceptable, whilst another will view that person as surly.

That said, if the sales assistant fails to answer questions about products when asked, for example, then that is quite easy to see so it's not impossible.