Theoretical question: Why don't new switches come with a default MTU of 9000

Hi all,

As far as my understanding goes, technology has advanced so much that network devices can push frames with bigger than a 1500-byte MTU. However, the norm is still stuck at the "default" of 1500.

My question is: If I change all my switching MTU to 9000 internally and leave the internet edges doing fragmentation, will that increase performance? In other words, is there any reason why the default MTU is not 9000 by default in the 21st century?