Image caption: A Dell Optiplex "ultra small form factor" PC motherboard. In a red circle are contact pads clearly designed for an M.2 SSD slot, with the words "M.2 SLOT" below it, but no slot is soldered.
Couple things to consider:
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Intel Core CPUs natively support m.2 as it's just PCI Express with a different physical connector. There's no need to add an "M.2 controller chip" like you need for SATA, you can wire the traces directly into either the CPU socket or the chipset.
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The traces are already there. That's why the board has pads for an M.2 slot in the first place!
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The slot itself is like two cents, and these boards are assembled by a pick and place robot so there's next to no labour involved either. This isn't like "oh well you didn't buy granite counters in your mansion and now you need to rip up the whole kitchen to add them".
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They had to deliberately make two different versions of this board, one with an M.2 slot and one without. Just to fuck over the people who didn't buy an M.2 SSD from them.
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It's infamously difficult to solder a surface mount component on by hand and you risk damaging other components and completely breaking the board, this isn't like the Commodore 64 days where you could grab a handheld soldering iron and replace a chip or a add mod wire relatively easily, so the barrier is so high that yes, for most people, they can never add an M.2 SSD. Having a professional reflow in the component would be prohibitively expensive, much more than just buying the M.2 option from Dell in the first place.
BTW: The slot further up the board is for the network card. The chassis doesn't have enough space to mount a standard SSD, and it's also the wrong "key" for an SSD which will prevent you from plugging an SSD into it. https://lemmy.ml/pictrs/image/6c2fd1d1-04eb-4765-8c5d-35a1fc9279c9.jpeg
Source: I own one of these machines, I've tried putting an SSD into it.
