Because a leading source of superheroes is circus sideshows. A merely strong man has not a chance in this world, where superstrength is possible, and the favored lion-tamer is a vague, sweet, innocent girl, as dainty and delicate as a flower, because that's more impressive. Natural job for someone with superpowers (not to mention gives them codenames and costumes in advance). Especially given that the metaorigin of this world is Mad Science, and the Mad Scientists' subjects are habitually lower-class not only because of their great number but their greater vulnerability. It's hard to get the upper-crust to respect you when you were a beggar kidnapped off the streets.
And once they leave the show, they are your classic vigilante superheroes -- which is to say, criminals. And then there's a supervillian or two.
Hmmm. I shall definitely have to give them supers who come from other origins. The circus mistress will have to conceal that she actually gives them the superpowers -- whether it's legal or not, they'd fudge something up if they knew she made them -- but with that secret, she gets high and haughty about how they would be the first to object if she held her workers prisoners and they should complain of the Association to Eliminate the Slave Trade.
In due course in this world, the police will come to terms with supers -- working out ways to work with them, which will involve some going to work for the government -- if only because supervillains can't be contained by conventional means. But it will take time.