Because the MacBook isn’t particularly short on space. The MacBook neo appears to have massive blank space blocks where the speakers are.
The 3.5mm jack is fine, there isn’t any need to replace it on the MacBook where you can afford to have both. On the iPhone it makes more sense to use the usb c for audio.
I'm guessing there are some key "user journeys" for scientific / industrial customer base, that involve using 3.5mm jack for something other than audio signal, and said customers would probably sooner change hardware suppliers than deal with dongles and all the problems of introducing USB into the signal path.