Because names sell tickets, and you can fill seats with names and mediocrity.
My Wings should have started the rebuild immediately after Lids retired and we didn't land Suter. That means dealing guys like Kronwall and Datsyuk. But who wants to see the Wings without those guys.
With older guys you probably aren't getting a top 10 pick which means you are unlikely to get anyone that can step right into the lineup. That makes it really tough to sell to a fan base. You are likely getting an asset that is a few years from making the NHL.
The NHL is largely a gate league. You need people in the stands. Hockey without skill is borderline unwatchable.
Also rebuilding is/was a total crap shoot. Again see the Wings. We still haven't seen a top 3 pick despite being at least a top 3 bad team on paper for most of those years. It can be over in 3 years or it can take a decade. Most owners would rather rake in the mediocrity money.