That's because they keep old stuff that's retarded and replace the rest with freshly invented retardation, rather than the other way around.
We still have HP pools, HP inflation on level ups, nonsensical effects of attributes, often AC conflated with dodge and so on.
Yeah, I think I've bitched separately about each of these in various threads here.
As you pointed out, strength has almost nothing to do with melee combat.
It has, but sure as fuck not for "hurr I strong I swing moar DAMAGE!!!1".
Once you start binding each other blades, grappling, etc. you'll find yourself in a rather helpless position if the other guy is similarly skilled but stronger.
Well, sword "binding" is a bit on the Hollywood side of things, if you watch Lindybeige or Scholagladitoria type videos on youtube or better yet, watch actual sword fighting on youtube, you really don't see much of that. Not saying it can't happen ever, but in a fight where it's all about lightning fast and lethal strikes, you don't really want to commit yourself to some relatively pointless pushing movement that your opponent could easy exploit.
Grappling's importance is also overstated. For most of the Dark Ages/Medieval Times period on which typical fantasy is based on, people did not use plate armor, which only became common in late Medieval Times. And with lighter armor and sharp weapons, grappling would be suicidal. Sure, it might be useful occasionally under certain circumstances, but a very niche thing. And even once plate armor came into play, one, only very wealthy combatants could afford it, so it was fairly rare, and two, much like Brasilian Jui Jitsu today, you really don't want to put yourself into a position where you are rolling on the ground with someone or entwined with them, because their friend could come up to you and easily kill you. So a much more practical response to plate armor was using weapons like maces and warhammers. Finally, while grappling does require some strength, it is much more about technique, as the skinny Gracies winning against massive muscle bound dudes in the early MMA showed.
Just because they didn't lift weights, doesn't mean they still weren't freakishly strong (Cus D'amato for example built Tyson's punching power by making him use heavier than norm punching bags since early teens).
It's not strength you are talking about. Tyson was a great puncher because he could put his entire stocky frame into every punch, a very complicated coordination of total body movement that most people can't do as well. To illustrate this point, if Tyson in his prime entered a bench press contest, he might have put up some pretty shitty numbers, but that in no way prevented him from throwing amazingly hard punches.
Small guys beating up on people thrice their size and weight (who've had similar training) with super technique is the domain of anime and Holywood (and well maybe early days UFC when no one knew how to defend against BJJ). In reality, weight classes exists for a reason, a run of the mill heavyweighter would crush Mayweather (one of the best in history) in the ring, let alone outside it. The difference in height, reach and mass would be just too big to compensate with speed, footwork and technique. I mean, Ali certainly wasn't known for his plant feet punching power but put him in lower weight class division and he would have knocked them around silly.
Weight classes aren't based on strength, they are based on weight, as the name implies. Using technique, heavier fighters can put more mass behind their punch, but they could actually very well be physically weaker in the sense of having worse bench presses or arm wrestling for example. How many times do you see a larger fighter with a beer belly against a smaller fighter with massive biceps? The latter is most likely stronger, but the former can throw a harder punch if they are both similarly skilled, because mass > strength in striking, as long as the guy is not a complete fatso.
Now granted, things could be very different with medieval weaponry involved but I imagine there's a good chance that two heavily armored combatants would eventually start grappling/wrestling each other.
See my reply to Draq above, about the prevalence of grappling, but even aside from that, that is not how RPGs model strength. They don't give you a damage bonus during grappling, or part of the time, higher strength gives you a constant damage bonus, implying it's related to the strikes themselves.