Because these movies take place in a different universe, and the two Keaton Batmans (Batmen?) stand alone.
This isn't how most people see the timeline, but we're pretty sure that's just because "reboot" wasn't a word in 1995. Batman's been officially rebooted twice since Clooney battled Schwarzenegger on a diamond-powered spaceship, so is it really so crazy that they snuck another one in there when we weren't paying attention?
This Universe Is The Same One As Beetlejuice's
And if the Burton batfilms are one universe, it's likely (or at least possible) that it's the same universe as Beetlejuice's. Just look at the fashion:
As you'll see in a minute, Beets dressing just like Max Schreck helps our theory even more.
Or the interior design trends:
No way there's more than one person who thinks that looks cool.
Or just take a look at this shot of Beetlejuice's home, from the opening of the Saturday morning cartoon series: