Well put. Since both modes lend themselves well to different situations, why not come up with combat that lets you switch them on the fly? If balance is an issue, you could disable certain things/abilities and add incentives to prefer one to the other depending on the context. Again, it doesn't have to be a tabletop simulator.
A compromise like this often leads to either one or both being sub par. Two examples: Arcanum, which was a complete failure in that regard. Neither TB nor RT worked well and combat was the worst part of the game. Then there was Fallout Tactics, which actually pulled it off although you really had no reason to use real time unless you just wanted to dispose of a trash mob quickly.
Well, having a single combat mode doesn't make a game successful per se so I think your sample size is pretty small. It's Larian we're talking about, don't you think they have what it takes to get hybrid combat right?
Well that was just the two that came off the top of my head. Arcanum had two systems because they had to shoehorn real time for multiplayer due to publisher demanding it, and ran out of time to implement in properly. Thus real time sucks and makes the game trivially easy for casters as you can just spam low lvl spells as fast as you can click while controlling all other classes in combat is tedious. I think the reason for two systems in Fallout Tactics was the same, but they actually pulled it off pretty well in that you can play the game in real time, it's just a bit of a clusterfuck if there's more than a couple of enemies. I remember setting up ambushes in real time was very satisfying.
Point is, these games had two combat systems because of external pressure to add multiplayer, not because the developers felt that adding two combat systems was actually beneficial to the game. It's naturally better to have only one combat mode and spend as much time fine honing it than having to share that time between two combat systems. I'm sure Larian could pull it off if they had infinite resources to pour into combat systems, but that is rarely the case in game development (nor will infinite time and resources necessarily mean that the end product will be better).
If you can think of a similar RPG with real time and turn based combat systems that are actually both well implemented and support each other, I'd be happy to hear it. And before you say it, VATS doesn't count since it's hilariously broken and makes the new Fallouts trivially easy.