For everyone other than people on .gov, .mil and .edu campus networks, the internet was dog slow anyway, so fork() and exec() wasn't a problem. Given the low traffic, it was probably also better for overall memory utilization. Memory was more precious than CPU cycles, and fairly limited if you didn't want a lot of swapping.
At the time, most stuff worked like that from inetd. It would listen on lots of ports and fork off the right program for the incoming connection, then that process would exit when it was done. Long running daemons and lots of processes weren't as common.
The common pattern prior to that was people logging in, reading already synced newsgroups, editing stuff, compiling, logging off, etc. Transient processes that went away.