The reason why this is not being implemented is because someone can take advantage of the incentives and install a lot of full nodes just to collect the profits. While that isn't a bad idea, imagine nodes being too centralized that the purpose of peer-to-peer relay of things is defeated once it reaches the hub of these nodes owned and operated by a single person/entity/organization. It's not going to end well, and there might be rare cases wherein nodes can go 'rogue' and disrupt the flow of transactions within the network.
yeah i see that as the main issue the thing is as more people join that 3% or whatever number would be split amoung more and more people until the finacal incentive is mute the issue is we dont really know how many of those nodes are owned by diffrent people but i think for chains larger then bitcoin like ethereum there needs to be some system that allows for terrabytes of data