It's not.

It's unenforceable to have clauses with no remuneration. So they can't compel you to pay 6 months of salary if you quit without notice, for example.

But these clauses with the big banks do have remuneration. In exchange for you signing this, you are guaranteed pay when you leave or get fired.

It's super common in trading firms (or trading businesses within banks) to have forced paid leave between jobs. For example, if you work for a hedge fund, you can't quit and immediate work for a competitor: you'd be able to take your knowledge of active trades/positions and bet with/against them. So you sign an agreement saying you won't work for 6 months (or longer) between jobs, but the old job keeps paying you.

All the big banks/hedgies are incentivized to also honor the system by not hiring you before your leave period is complete. Why? They don't want their own ex-employees poached during their leave period to have their positions wiped out.