It's what happens, because babies are put in their cot to sleep on their back so when they are awake some of us naturally carry on and put them on their back on a bkankie on the floor, under a playgym arch thing etc.
tummy time, a modern invention, is advocated to make up for different sleep time routines.
rolling over must be separated into two different categories:
1 - roll from tummy to back, which is fairly easy and babies who spend time on their tummies can achieve this first by accident around 3-4 months
2 - roll from back to front, which is much harder and a "conscious" effort.
look at animals. once on their back, they are extremely vulnerable and it is also a sign of showing weakness.
and some are unable to roll on to their front!
babies might start rolling from back to front around 6 months, but it takes time to master that skill, loads more muscles are involved and needs coordination!
OP
I have 6 children and none of them liked tummy time.
consequentially they never had a chance to accidentally roll from front to back then progressing to practise this.
DS1 rolled from back to front twice at 6 months, next thing you know he learnt to sit up and crawled
the others were similar.
He is now almost 13 and absolutely fine. You have nothing to worry about, your baby's development is not delayed - and if you start putting on him on his front to play and look around he will probably roll over soon.
good luck