Because success is pretty much a personal concept, meaning that it's unique to every single one of us. It becomes difficult to frame it as a one size fits all idea.
Interestingly enough there is one aspect of success we can all agree on. Personal satisfaction is probably the one common characteristic. Think about it for a second, regardless of income, regardless of your career, the path in life you have chosen, if you are satisfied you could accurately call yourself successful.
Maybe that is the trick, if any, to spend some time thinking if you purpose, if your values align with the environment you live in, and if this environment supports them.