There are multiple but the main one is that all communication related to the activity of the bank needs to be recorded and be auditable. There are limitations for expediency. For example, only phone calls with customers or peer organisations are usually recorded. Most phone calls aren't as it's very expensive and not massively useful (although AI may change the calculus on usefulness). But all email, IM, BBG chats etc are recorded.

And it's as pervasive as it is primarily because of the scandals of the credit crunch. Similar controls existed before then but they were less pervasive and not as strictly enforced.