Because purists (most of whom hodl very little BTC) don't want to risk what little they have. Check their Zaps to know if they spend their Bitcoin on value or not.

With loans the only factor that limits is the LTV and interest. Say you have 1 coin, you are risking that coin AT interest AND only getting half of the nominal USD value in return. If that is an acceptable risk because you make more than you spend, do it. If you are just waiting for your bitcoin to get you a lambo, I got bad news for ya.