Ignoring the harsh words, I think the problem you are stating is that 'freelancing service providers commission are high' and you think flat fee could address that.
I guess by charging commission, they ensure that the fee is paid only when the work is done. But if it's a subscription, even those who don't get a work needs to pay each month and soon freelancers would leave for other platforms where they have to pay only when they get paid.
Comparing a consumer service like Netflix just because it is a subscription service seems just wrong. But, I will leave someone more knowledgable about business models state why it is so.