Furthermore, he could easily earn big bucks with his super-powers as Spiderman. How did he end up as a poor photographer?
Peter Parker tried earning money as Spider-Man. While being rooked out of his earnings, he let a thief steal money from the person who cheated him. That thief later killed his uncle Ben.
He was so traumatized by that, that he swore to use his powers only to help people and not for personal gain. He spends most of his free time as Spider-Man, doing heroic things. He doesn't have time left for a real job. Instead, he takes pictures of himself as Spider-Man and sells them to J. Jonah Jameson's newspaper. Since he is Spider-Man, this is easier for him than it would be for another photographer. But he still gets paid as other photographers would.
For the amount of effort he puts into his job, he gets paid well. He just doesn't put much effort into it. So he's always broke. And he can't really put in more effort, as there's only so much of a market for Spider-Man pictures. And that's the only kind of photography that he's efficient at doing. For most other tasks, he has few advantages over other photographers.
It is of course quite possible that he could both make more money and do more good as a scientist or engineer than as a super hero. But his own hangups won't let him do that. He still feels guilty about his uncle's death. He concentrates on directly preventing crime and death rather than indirectly preventing death through scientific achievement.
If that doesn't satisfy you, then it's a plot hole. That is the proposed reason why he works as a photographer rather than a scientist or engineer.