How shortsighted can you be? Say you got your way, say tempering had no limits and seasonal characters were forever. Fast forward 3 months, and you got the build of your Dreams, every stat is near perfect and you deal hundreds of millions of damage, crushing everything in your path. Season 4 just ended and season 5 is about to start, and it takes your godlike character all but 5 minutes to clear whatever seasonal content was added, then what? You go farm some more gold to try to turn that one last 49,5% stat into a clean 50%? You go kill Duriel again for that one last uber you still havent gotten for your collection, even though you dont need it? You stop playing because you ran out of meaningful things to do? Or do you make a new character and do the whole thing again? Oh wait, thats how seasons work now and you dont like that.
Seasonal resets are what keeps games like this alive.
And btw, yes, D2 had seasonal resets aswell, they just called it ladder instead of season, but the concept was the same: you start from scratch to keep things from going stale.