Two main reasons:

1. It’s about the leveling experience being different every season. MMO fans don’t typically understand that the fun in ARPGs (usually) comes from variety, whether that be different skill builds or different loot along the way making every playthrough a little different. Seasons also change mechanics and add new season-exclusive items to tweak the leveling experience.

2. New season-only items, cosmetics, and battle pass. Everything other than the battle pass should usually be available on the Eternal realm after the season ends, but you get it sooner on the new Season realm.