Because if they find out you can actually trade they don't want you to take money out of the "bucketshop fund" that everyone feeds into when they pay for evals and resets. This is their buffer to keep their front-end afloat.
Once they see that you can trade they want to make money from you and stop the hemorrhaging from said bucketshop fund.
So, why trade with them?
They scale you up. If you have access to multiple MM wouldn't you want to trade that, even with a 90/10 split?
If you're taking out $10k from them to "open your own account", why would you even do that just to swing a 1 lot, when you could manage 100-500x that and grow faster?
5MM * 10% monthly return = $500k. $500k * 0.9 = $450k. Were you really that smart for withdrawing $10k and going out on your own?
If you're worried about professional data fees and a measly $150/mo. you need to find another line of work my friend. Not being rude, just blunt.