nobody bu the regulators
SURE, they could put up their own payment systems... if they had a crap-ton of servers, a staff costing a few million a year, and complied with the 60 different versions of local legislation and security requirements regarding collection of payments, and taxes, and disbursement of said taxes.
Small devs DO NOT WANT that kind of hassle, nor even know the first things about SOX compliance, and the miriad of regulations tied to collecting payments. One company operating in one country, sure, and even THAT is a damned pain in the ass, and usually outsourced to a 3rd party web site.
Apple made it easy, everything stored on servers they have to operate anyway either for iTunes, or just to sell hardware in various countries. No credit card information to type in, no account information stored in the phone, easy. For this to work ,google;s going to need to get in the micro-transaction game, or users are going to suffer for it. Even carrier specific processing is cumbersome, and it;s going to have both security and developer issues.