The problem is mostly AI and encounter design which are both easier to do in turn-based models, which is my guess why we see often better combat in turn-based games.
Hmm...I think what helps make the classic RTwP games (BG and IWD) great is just that they do have good encounter design (for some reason people are obsessed with the trash fights in BG1 when the trash combat ratio is actually lower than in most CRPGs).
That is true but I think another underappreciated factor is that in order for rtwp to work you need to limit the amount of pausing that is needed to a managble level. I don't think this was intentional as much as byproduct of implementing 2e (that has limited to no active abilities for martials) and the engine being built for RTS gameplay and then retrofitted to work for RPGs.
I agree. RTwP is better suited for combat with many units that have few active abilities. But I think there is still some room for more complexity in RTwP than the Infinity engine games offered, without having to pause at every moment. And I'd very much like to see something like that. The problem is that this would require some developer to come up with their own system, which usually doesn't turn to well and doesn't sell since its not some popular brand. There are many mechanics that are much more natural in real time, such as channeling spells, interruptions, positioning, differences in attack and move speed, environmental effects, concurrent actions, etc.
Many of these things can be done in turn-based combat to some degree by using systems with more action points instead of just some low one-figure number of action points, but in the end its still less natural and constricted or turns into a book keeping nightmare to find the most efficient distribution. Although my main gripe with turn-based combat is typically alpha strikes and turn-order, which isn't as problematic in real-time where enemies can react immediately.