Don’t need to worry about canon or universe continuity
If you do combine the two lines of storytelling; television and film, then what you’re doing is creating a bigger web of continuity and canon to follow than you really need to. Right now there are Star Trek fans who will lie through their teeth to tell you “there’s no such thing as too much Star Trek”. That’s a lie. A bald face one at that.
There is such a thing, and the Marvel Cinematic Universe and Arrowverse’s are living proof of that. So many films and shows to follow just to keep up with the larger point, only to then find out you don’t need to follow the other properties because the team up stuff is the only thing that actually moves the plot forward.
That’s what Star Trek is about to turn into. That’s what Star Wars is turning into. Yeah, the Mandolorian is doing well now, but just wait until Disney+ has five shows and three movies all debuting in the same year. No one will care. It will happen, fan fatigue is a real thing and it happens all the time. No franchise, no fanbase is immune to it.
Keeping the films separate allows them to be their own thing away from the burdens of canon and continuity. New and fresh stories can be told without having to labor away to figure out how a line of dialogue may destroy the entire premise of Star Trek: Strange New Worlds.