The reason was literally to make the phones thinner. The 3.5 mm jack was the thickest component by far, and it has to sit under the screen.

Space is not a constrain in many flagship phones. Check this comparison of (unofficial specs) s24 vs s8 (0.5mm appart. S8 also had sd card slot)

https://m.gsmarena.com/compare.php3?idPhone1=12773&idPhone2=...

This is the datasheet of a generic 3.5mm It is thinner than that: 3.2mm.

https://www.cuidevices.com/product/resource/sp-3530.pdf

Just the tip is 3.2 mm. Ring and sleeve are 3.5 mm.

But phones aren't really that thin, especially with the camera lenses. Why didn't they just put the 3.5 mm jack on the other side from the cameras, protruding a little bit from the rear housing, exactly enough for the phone to rest stably on a flat surface without rocking?

That was the stated reason. The iPhone 14 is 7.8mm deep without a 3.5mm jack the iPod Touch (4th gen) was 7.1mm with a 3.5mm jack.

Additionally as other have stated there are thinner (2.5mm) audio jacks.

The iPhone 14 has a glass case while the iPod Touch 4th gen had a metal case. That alone accounts for a significant difference in thickness of the internals of the device.

Space, and water/dust proofing

There have been aweasome flagship phones with ip68 and 3.5mm audio jack

https://en.m.wikipedia.org/wiki/Samsung_Galaxy_S8

So, no. Waterproof casing is an excuse.

I went to (Edited) A Mainstream Chinese Brand now that Samsung dropped the ball. I dont get audio jack, either, but I have IR blaster and double physical sim

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.

"These days" that wasn't the case four years ago when they arbitrarily took it away. How would you know what the level of demand is? I would selectively purchase a phone with a headphone jack over one without, but that demand is meaningless without an option. Apple led the pack with the highest R&D budget of them all, with a premium smartphone. They, uniquely, took the headphone jack away while launching airpods.

You're right about there being no mystery there.

The waterproof 3.5mm jack connector isn't an R&D cost, it's just a standard component you can buy from Digikey; comes with an o-ring. A whole generation of waterproof phones had them, like the Sony Xperia.

They look like this: https://www.digikey.com/en/products/detail/cui-devices/SJ-35...

Yeah, that might not be the best example. Normally you’d want a double seal or really tuned compression to reliably hit IP68. And the o-ring adds a manufacturing step. And you have to design the case to properly compress the oring. And case assembly is harder because screwing up that interface will increase chargeback. And you’ll need to modify the audio test fixtures to add a jack. And the audio test has to include a headphone test. Any failures increases rework, so you gotta budget for that too. Probably need to add a cheap earbud to the box, so you gotta add a packaging step and a bigger box. And once you sell the phone, any failure within the contracted return window will increase chargeback.

Seems like a lot of faf. I tell you what, why don’t we wait for a bigger OEM to remove the darned things, and if it doesn’t hurt there bottom line we’ll just quietly scrap the idea?

And that’s how we got here.

You're implying it's because of cost-savings, but OEMs still typically offer 3.5 mm jacks on their lower-end models (even waterproof ones, like the Xperia AceIII). It's the expensive flagships they've removed them from. I think a better explanation is that high-end models are fashion items while lower-end models are functional items, and it's fashionable to copy Apple.

I've read speculation that because the jack can pick up electrical fields, you have to design the phone around it. Otherwise the user will hear buzzing in their headphones.