Because nobody yet built a good implementation of the idea. There was Flattr, but they pivoted from their original idea because they couldn't make it work for

Because nobody yet built a good implementation of the idea. There was Flattr, but they pivoted from their original idea because they couldn't make it work for some reason. It can't be implemented on the level of individual sites - the solution needs to be universal, like Google adding a 'tip' button to Chrome + Play Store payment integration.