I think you've missed the point a little or are perhaps are being slightly obtuse. Of course it's POSSIBLE to make a phone with waterproofing and a 3.5mm headphone jack, and of course it POSSIBLE to make a modern thin phone with a 3.5.mm headphone jack taking up extra space...
The point is, it's simply harder and more expensive to make a highly waterproof and slim phone, while having to have another large hole in the phone which requires a long connector to fit inside it. And when you consider that extra expense and R&D required to design the phone shell and internal architecture, versus the actual demand for wired headphones (basically zero these days), it's obvious why major phone makers don't bother to include it.
It's simply a matter of cost benefit analysis. Lots of cost, very little benefit. No mystery.
The point is, it's simply harder and more expensive to make a highly waterproof and slim phone, while having to have another large hole in the phone which requires a long connector to fit inside it. And when you consider that extra expense and R&D required to design the phone shell and internal architecture, versus the actual demand for wired headphones (basically zero these days), it's obvious why major phone makers don't bother to include it.
It's simply a matter of cost benefit analysis. Lots of cost, very little benefit. No mystery.