According to this article about 1/3rd as many movies are made in LA since the 90s. They were only making 30% of films in LA in the 90s so your experience still matches with the data but having 2/3rds of the local film production move elsewhere is significant.
Before the 1990s CGI did not exist yet. You really had to film stuff outside.
Seriously? Up to a certain point filming in studios and backlots was pretty much the norm.