In a lot of those cases, the company has decided that the lower tiers do not exist, because they lose money, and part of the point of the sales process is to weed them out. (And even if they do have lower tiers, they don't want to hurt their actually profitable higher tiers by providing price anchoring.)

Depends on the product. Not all products are built for these types of trials. If a product needs more heavy lifting to show value then you are more likely to have a sales team. And just because you can evaluate that complex product by yourself doesn't mean that happens for everyone else.

Selling, by the way, is not just demo'ing the product. It's also navigating legal, finance, security, end users who don't know as much as you do, etc. As a buyer, it's sometimes easier to have your rep help you identify what the quickest path is. Lets me move faster, anyway.