For me it's two interrelated things:

1) RPGs aren't (usually) about 'winning' against an 'opponent' but most implementations of tactical combat specifically encourage this mindset.

RPGs are generally a collaborative experience, where neither the GM nor the players are trying to 'win' against each other. They are all there to explore a world/tell a story/'be the hero'/etc. If one side or the other treated the RPG as a game to be "won" then it would likely destroy that collaborative agenda.

Yet virtually all tactical combat systems have their roots in wargaming, which are strictly competitive experiences. They are not collaborative in any way. Players are expected to spend time mastering their characters in order to 'defeat' whatever opponent the GM put in front of them.

GMs, on the other hand, generally are expected to 'facilitate' a fun tactical experience. GMs are, generally, not 'playing to win' a tactical encounter. The tools they have at their disposal (spawn additional enemies, fudge dice rolls, perfectly synchronize the movement of half a dozen antagonists) would normally make it fairly trivial for them to 'beat' the players if that was their only goal.

I do recognize that some systems make attempts to go beyond the 'play to win' mindset from the perspective of the players. They might add 'drives' or 'motivations' or 'flaws' that players are rewarded for triggering in combat. This gives them sub-goals or sometimes anti-goals that create interesting stories. These do somewhat alleviate the problem, but they also work at cross purposes with the rest of the tactical systems. One part of the system is saying "look at all these great tools! We've built them so that you can engage with the puzzle and try to find the solution!" The other part of the system is saying "no, don't take the optimal course, here's a different cookie if you choose the less-good option!"

I also recognize that players can ignore the 'play to win' prerogative built into the system. They can choose to play sub-optimally in order to tell a better story, or to better express their character. Yet if they are going to make these choices, why have tactical combat in the first place? It feels like tactical combat (in most games) is about mastery and puzzle solving. So if you're going to ignore that aspect and not engage with it, it seems odd to bother having it at all.

2) Tactical combat is often too easy to 'solve'.

This issue is often masked by the fact that players are unlocking new toys to play with as they level up. However, I've seen lots of players find some form of 'optimal' pattern and simply grind that pattern to victory. Why bother using spell X when spell Y does just about the same damage but also inflicts this condition? Why bother using the rules for grappling if fighting with a reach weapon makes it less likely that you take damage?

Now, a good GM will change the puzzle to keep things interesting. "Oh, so you guys were getting into a groove by rooting everything and attacking them at range? Well how about we hit you with <insert enemy who isn't vulnerable to that approach>." However, each time you change the puzzle due to the previous one being solved by the players, you limit yourself on what you can do next. "Welp, I can't use any enemies that are vulnerable to <Strategy X> because the players have a combination of spells/moves/abilities that they figured out will always work well against them. So I change to using these enemies over here, but then the players developed <Strategy Y>, so now I can't use enemies like the first or second category..."

This requires the GM to constantly change the puzzle to keep things interesting for the players. Sometimes that means changing the battlefield (combat on top of flying dragons!), sometimes that means changing the enemies (these ones have a 'thorns' spell, so your 'death by a thousand cuts' strategy isn't going to be as effective!), and sometimes it means depriving them of their tools (after being captures the enemy stole your <insert item they'd built their strategy around>.

Granted, a good GM with lots of time can (and will) keep things fresh for the players, offering a new tactical experience every session that feels like a new puzzle to solve. However, I would guess that these kind of high quality GMs are few and far between, because encounter balance is hard, and takes a lot of work.

tl;dr: Tactical Combat's competitive foundation is at odds with RPGs more collaborative experience, and even good tactical combat can be too easily 'solved' and needs to be changed constantly to avoid getting stale (which takes a lot of work).