I mean, if we're talking fantastical, the idea that a Fighter can in well under six seconds (from level 5 onwards) potentially kill or maim four different people in four different directions with four different attacks (each potentially having a different Battlemaster rider on them!), rising to 6 at 11 and 8 at 17, whilst also moving up to 30ft and not even having used up their Bonus Action (!!!), that's insanely fantastical, especially if they're using say, a greatsword or a greataxe or god help us, a maul (which I daresay is 100% physically impossible to do in six seconds, even a sweep/turn IRL is going to lose so much speed after the first impact it'll do next to nothing to the rest).
(To be clear I actually think Fighters can make too many attacks per round, I think that's basically a bad hangover from 2E and we should max out at 2 attacks + bonus action and most classes should get that, just Fighters should get attacks doing stuff like load and loads more damage - I mean, how come Paladins get to hit hardest in a single swing? - or whirlwind attacks or the like)
So this is completely inconsistent. It's not really about what needs a magical explanation, it just a selective "Well this impossible thing is okay because I personally don't imagine it to be impossible even though actually if I think about it for even a second it obviously is but that impossible thing should stay impossible even though it's not really any less plausible and is even more of a genre trope!".
D&D "but it's not plausible!" stuff remains crippled by the "feel free to jump off a building" nature of HP too. You don't even get stunned or risk death by massive damage in 5E lol. Let alone break anything. You literally just get prone'd if you jump a survivable height, even straight on to granite!