As a part of a prop firm a trader still gets lower commissions and likely more leverage than retail + a firm may offer a ton of other benefits: better support from broker or even directly from the exchange, tax-efficient re-investment of profits, free health insurance, a small salary for "bad" months, office to come to where one can interact with other people etc. It's not cheap and it's a big operation to get all of this for a stand-alone trader. Aslo it's not everyone's cup of tea to run a whole business to support trading. Naturally, beginning traders who have made a jump to significant profitability shop around for a better deal but then top prop firm are there in part thanks to willing to offer deals that keep the top talent.
If you ask me, they should. Unless they are getting some worthwhile benefits they can't get elsewhere, OR they have a strategy that doesn't work at more traditional commission rates. If you are making money trading 50,000 shares for twenty cents, there's a good chance that won't work paying a per share rate.
the easiest way to make money in wall street is the management fee and commission model. the broker or management company has NO RISK. a fund with 100 million with 2% managment fee is 2 million in fees. brokers have no risk either. prop firm model is just like retail trader playing the market. prop firm have risk and risk is 'participating' in the market. prop firms or market makers are 10:1 leverage and mostly daytrading..they don't hold positions overnight. some market makers do but it;s hedged. reason brokers/banks got into prop trading was it's more profitable but more risky..and you are always taking the otherside of the trade of the client. the answer proptraders are just retail clients no different prop traders are market participants.
Commissions. The last prop firm I was at was charging me about $3,000 per month in commissions and they had hundreds of people there. (You do the math). You also pay your own SEC fees at a prop firm (currently $19.20 per million dollars sold) You can expect to pony up at least a few hundred dollars per month in SEC fees at a prop shop.