The Galaxy class starship was designed with the ability to separate the saucer from the stardrive section, so that the “floating city” part of the ship could be left somewhere safe while the rest of the ship galavants off to do something risky. We see this happen precisely once, in the season one episode Arsenal of Freedom. We also see saucer separation deployed for a handful of tactical and or emergency uses (such as against the Borg in The Best of Both Worlds, or to escape the breaching warp core in Generations).
So, this seems like a useful ability to have, and the Enterprise is constantly being sent into dangerous situations. Why not use this ability more frequently?
From a real world perspective, I think they just found it slowed down storytelling too much. I think the idea originally came from Gene Roddenberry and they had always been trying to make it work.
Ironically in Best of Both Worlds, it’s a lure to distract the Borg from the rescue mission, and it wasn’t like they offloaded the kids and civilians first. Pretty high stakes gamble on Riker’s part.