Being able to easily win battles doesn't mean you understand the combat mechanics; it just means that the game is so easy you can win with left-click and autoattack. Take for example the initiative/weapon speed/attacks in a round debate that has been going on for years. It's because a) the manual says one thing and b) the game does another while failing to provide relevant information.
1) RTwP is bad because the baldur's gate manual contained incorrect information, or was that an unconnected example?
2) You don't have to have perfect clarity of knowledge to
understand what is happening. Ask a professional at any sport the physics underlying how they do what they do and most won't be able to give you the basics, that doesn't mean they don't understand what they're doing in a relevant sense. Most info in BG is clear enough for a teenager to pick up (a +1 sword adds +1 to your attack roll, something with lower AC makes you harder to hit, this spell does that, when you get hit while casting a spell you have a chance to be interrupted, etc.).
With TB you have the time and information to take informed decisions. It's as simple as that. With RTwP you have to guess where the enemy will be when your casting animation ends and to me that's simply not that fun. A TB system allows you to build far more interesting encounters, because you can place monsters and give them abilities counting on the fact that the player will always have all the information he needs to make choices. That's it, at least for me.
The problem is not just "understanding what's going on", the problem is that they have to build encounters understandable in RTwP.
To the bold part: yeah exactly. That's the difference and that's what some people (including myself) enjoy. If you don't enjoy it, that's fine: don't play RTwP games! As to the second part, I don't think it's true. I have had more fun with challenging encounters in BG1/2/ToB and IWD1/2 than in any other game. And I actually love tactical turn-based games; always play them on max difficulty and enjoy doing so, but none have ever matched the complexity and challenge of the infinity engine gameplay + encounter design (with AI mods, but we're talking about theoretical RTwP system).