In this particular case, a combination of factors:
The company must ensure that all work-related communications for the traders (the people directing billions for the organisation and its customers) are recorded, not just recording all phone calls and emails, literally everything. They like to have these people work onsite for this because of course nobody can ever be trusted and nobody ever goes to lunch…
Management is all boomers - literally can’t get their heads around remote work. They think that if you aren’t in the office suffering then you are skiving. It’s literally about an identity and conformity.
Management is completely focussed on traders - 'if they all have to be here then so must everyone else’
They pay exceptionally well. Absolutely a horrible environment but if you move the needle at a hedge fund then they will pay you more than any other type of employer.