Because classes change over time, even if you are playing solo.
Just look at the Mage Tower for timewalking, they had to completely re-balance the thing because your kit isn’t the same as it was in Legion.
If they were to leave the tower up infinitely, they would have to look at the balance of the thing every time they wanted to implement class changes. Blizzard doesn’t want to have to monitor the balance of the mage tower for perpetuity, so they made it a limited time thing.