There is a big difference between submitting something to an auction site, them selling and then skimming their commission off the sale price and us submitting to an agency. The auction is a once off scenario, payment for services rendered. The Agency has a long term commitment. Just because their income is 70% to 85% does not make that amount profit. There are a lot of expenses that come out of that income, including but not exclusive, insurance, IT equipment, Software, various personnel that have to be paid, utilities etc (the list goes on), and last but not least the shareholders who provide income for the growth of the company but expect something in return (dividends). All this cuts into the income received. They are probably doing well to get 15% profit. The reality is that after you expend time and resources on your images/video etc, you submit to an agency and then you let them do all the work .... that is what you are paying them for.
I knew an author way back who had a very successful book, we knew how much the book cost to buy (approximately $30), he then said he made 20c royalty per book. This was way less than 15%. But just like us and stock images he did not have to do the work once it was in the hands of the publisher.
If you feel that the compensation is not worth the effort then this profession is probably not for you.