Most people here have said it's about the money, a few said it's for motivation becaue "they like me they really like me!"
I'd argue the fundamental reason is because no one wants to waste their time and effort. If you're pouring yourself into something and getting nothing back then what's the point? when you put effort into anything you do so with some end goal in mind which is your reward if achieved. Money and praise are both material rewards and material rewards are all that really matters to anyone. "But Terra, emotions aren't a material reward!" Yes they are because they are caused externally. Or if you'd like you could see it as getting high off of your own body's chemicals which I class as external. This gets into a bunch of philosophy and practical thinking about how you define what is internal versus external and thus what counts as "you" vs "everything else" and what your locus of control is etc. Which is an expansive topic well beyond the scope of this one.
I don't think it's possible to not be affected by external factors but you can have different definitions of things like "effort" and "reward" so that it appears you're unaffected to other people. For example a person who is putting effort into their comic to become more technically skilled t the comic creation process may not care so much about number of pageviews. They might care about quality and of comments however. Someone trying to make money may just care about blanket praise and popularity and thus pageviews mean a lot to them. These things are all internally defined and fundamentally come back to some very basic expression at the end.
The player archetypes for video games are a sort of good example of this, Whether you're a killer or achiever or socializer, etc. Or rather what mix of it all you are.
