Relevant thread. Unfortunatly links within links, but should be enough to get a general picture.

Very few developers do indeed allow you to purchase their games straight from them. But considering most don't even have their own homepages nowadays, these are very very few, and you have to manually check them.

Umm… that depends on the developers. Some do sell games directly, while others don’t. The main issue is that if a developer wants to sell a game themselves, there are significant costs and liabilities involved. They’d need payment processing, a delivery system, knowledge of legal requirements across different countries, support staff and so on.

There’s a reason why so few developers sell directly and instead use stores, even though the store takes a cut.