Whenever money is involved, greedy people and content farms start appearing. That would not benefit the Fediverse in any way.

Integrating it into the client apps means that fake apps will start appearing to steal wallet keys. That already happens with normal wallet apps.

The Fediverse is supposed to be free and volunteer run. Tipping is normally implemented by the instance admins on their website and not everyone wants to deal with wallet keys and conversion/selling of cryptocurrency. Such a thing shouldn’t be a part of the Fediverse, but a decision every instance makes for themselves.