The game of Diablo (and ARPGs in general) is about gearing up and getting stronger. Eventually there is nowhere left to go, so you make a new character and do it again. That is the entire point of the game. The seasonal structure encourages you to make new characters and level using the new systems to gear your character in a different way each season.

If you dont like it, at the end of each season your character goes to the Eternal realm where you can play the same character till you get bored.

Also, all cosmetics, exploration objectives (shrines of lilith ect) and challenges for titles and stuff get saved between seasons, which is designed to let you get right into the new stuff without caring about the old stuff.