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.
Doesn't sound like the 'cheap small computer you can run your hobby electronics project on' that the original Pi used to be. It is not as cheap and a power hungry beast, still small, though. More and more like a PC and less and less a small cheap embedded platform. For some people it is a plus (I guess for most people here), for some not so much.
I tend to build my projects on Raspberry Pi Pico now, but sometimes I would need something more powerful and Raspberry Pi 5 will be too much.