It is not 50%, even if you are allowed to post passive orders (quotes) and pocket the spread (instead of loaing it) commissions will eat a lot.
Effectively, brokers and exchanges have almost infinite ROI: with small and fixed investments in infrastructure and marketing they take _percentage_ of your traded volume.
And shady trading rooms will screw you on commissions, spread, high margins on volatile instruments, and in very unlikely event if you manage to cash out more money than deposited, they will hold your account for "review of illicit trades".
But even on real and legal exchanges far less than 50% of capital wins, I assure you.