Because 3.5mm jacks suck. 6.3mm jacks are much more sturdy and can be easily mounted on 6mm or even thicker cable, which can also handle much more use.
Flimsy jack and thin cheap cable cable is asking for trouble during performance.
The only plus of 3.5mm and smaller 'phone jacks' is their size and in many applications it is much less important than reliability.
Because 3.5mm jacks suck. 6.3mm jacks are much more sturdy and can be easily mounted on 6mm or even thicker cable, which can also handle much more use.
Flimsy jack and thin cheap cable cable is asking for trouble during performance.
The only plus of 3.5mm and smaller 'phone jacks' is their size and in many applications it is much less important than reliability.
Don't adapters exist?
They do and make reliability even worse.