Because Satoshi didn't think it was necessary and didn't code any method of doing so into the protocol. Still, if you really want to, you could set up a node that refuses to accept connections from any peer unless they pay some bitcoins for that privilege. What will your expesive node offer that peers can't get by ignoring you and connecting to free nodes instead?
The question was not made because i want to get paid for running a node but to see if giving incentive to nodes would solve the decreasing bitcoin nodes trending at the moment...
He was providing you an answer in the form of a question.