Because most developers are presumably experts in programming* and whatever the domain of their app is* (game, drawing program, whatever) and not experts in the psychology of pricing (unless it's a pricing app I suppose!).

Now so what? They could allow you to pick a predetermined price or type whatever integer you want into it. But I assume they think consistency makes for a better buying experience, and poor pricing choices are a kind of externality that affects everyone.

I'm slightly dubious of this "externality" really being bad but in this case I'll give the horde of Apple marketing researchers the benefit of the doubt.

* This is HN so the obvious snark has not been inserted