"ATC maintain a separation in the sky, (surely it would be possible to arrange the same on the ground)."
That's because normally you can't see the other guy, and you are travelling a high closing speeds. It's also in 4 dimensions. On the ground you are slow, and should normally only be concerned about threats in front of you that you can see and are using Mk.1 eyeball x 2. ATC has enough on their plate to play wet nurse on the ground. I don't know about all types, but you can see the wing tip of B738 from the pilot seat. OK, the threat might be behind you in a turn, but you have seen it and assessed it before the turn. When in doubt there is not doubt.
For ATC they do a wet nurse thingy at LGW & LHR and a fantastic job they do too: follow the greens at night. Fantastic, but then they are shuffling big lumps of aluminium in a maze of taxi-ways with many pilots who are very unfamiliar with the puzzle. Dublin is hardly in the Krypton Factor realm of challenges.
I do not wish to hand over more control than necessary to an outside agency. The captain has to maintain control and responsibility as a much as is sensibly possible. I see too many youngsters just follow an ATC instruction without assessing if that is what they really want to do. Head teacher gave an order and there'll be no dinner less you comply attitude. It's sad to see guys aim at a single Cu on a clear day just because ATC gives a radar vector. It's sad to see guys allowing ATC to suck them in hot & high without saying No. That's a whole new topic, so sorry for thread diversion.