There's also a matter of in-flight trajectory adjustment. While doing retro burns to prepare for reentry, you want your trajectory towards the landing site goes as planned, so having the service module still attached until just before reentry could allow the crew to make minute adjustments and corrections, because once you start the reentry, there's no turning back, no controls and no more corrections. If you drop the command module too early, you're basically gambling your fate. It's just you returning to earth like ICBM warhead purely on ballistic trajectory