I think one major reason is that endings are incredibly difficult to write.

I don't think it's that difficult when you already know how the story ends. The real issue here is that everything depends on how the audience reacts over the course of the seasons. Major studios can ruin everything in the pursuit of more money, causing creators and writers to lose the essence of their work.

The longer a series runs, the more expectations it creates.

This is one of the main reasons why a TV series needs to start out knowing what its ending will be. The essence of the project must reflect the creator's idea, rather than satisfy the audience's expectations (which is obviously something quite important, but takes a "back seat" to be honest).