Deals involve a tremendous amount of back-and-forth, changing deal terms, changing financing details, etc. Bankers could simply go home at 6 pm and get to requests the next day, but that would make deals take longer. While a deal is in progress, it's fragile. Parties can decide to back out, prices can go up, third parties can swoop in. That's the sort of thing that causes shareholders to get pissed, lawsuits to get filed, etc, and is generally to be avoided. Also there are the usual deadlines of corporate life: we want to get this off the books this quarter, we want to get the merger done this fiscal year, etc. All that results in C-suite execs picking bankers who will turn things around as quickly as possible.