Of the purchase price that the end-user pays, the retailer has to pay tax. That knocks off a variable percentage. It would be 20% in the UK.

There's also the cost of selling through Steam / Google Play / Whatever - typically 30%.

I assume the developer has some professional expenses - an accountant at a minimum, probably a lawyer, certainly insurance. Maybe they also have a PR team, advertising, and the like. I don't know whether they pay for testers, translators, and things like that.

Then we get on to things like buying a new development machine, going to tech conferences, taking an educational course, backups, and all the other things that a business needs to spend on in order to be effective.

Maybe a profit margin of 10% is unrealistically low - but developing software has legitimate costs. The margin is never going to be 100%.