Why not have a big default L2 MTU on network equipment?

I know what MTU is, I know what jumbo frames are, I just don't get it why vendors don't turn jumbo frames by default. What would be the downside? The only thing I can see is jumbo-traffic that's gonna be dropped on some device that doesn't support jumbo frames is gonna traverse the whole network before dropping. Is that the only reason?

Edit: guys, I'm not asking about the benefits of turning on jumbos on the servers. I'm asking about the benefits of not turning on jumbos on the networking equipment.