PoE is not obvious to implement (take it from someone who has done it with a fair share of mistakes), uses more expensive components that normal ethernet, takes up more space on the board, makes passing emissions certification more complex, and is more prone to mistakes that ruin boards in the future, causing support/warranty issues. In other words, a bag of worms: not impossible to handle, but something you would rather avoid if possible.

A full-module add-on in this power class is about $7 at 1,000 unit scale [0]. It would be around $3 with your own custom PCB design in terms of BoM addon at scale. That’s power only. Add another dollar or two for 10/100 PHY.

The trick is as others have said in what adding it to your design does in terms of complicating compliance design.

[0] https://www.digikey.com/en/products/detail/silvertel/AG9705-...

PoE power supplies need to be isolated (except in rare exceptions) and handle much higher voltages than common USB-C or wall wart power supplies.

They have to use a transformer and a more complex control strategy, not a simple buck regulator with an inductor. PoE inputs need to tolerate voltages several times higher than the highest USB-C voltages, so more expensive parts are used everywhere.

Ethernet is already one of the most expensive standards because you need magnetics for isolation. Adding power on top of that is genuinely expensive.

Whenever you combine two things into one, the complexity and cost go up considerably. A regular coffee machine is pretty cheap. Add high pressure so it can make espresso and it gets considerably more expensive. Add milk so it can make cappuccino, again more complex and expensive. The same holds for electronics. Isolating power when it's alone is fairly straightforward. It gets considerably more tricky and hence more expensive the moment you want to place any kind of a meaningful data signal in its vicinity.

I’m sure the other commenters are right, but I’m guessing market segmentation may play a role here too.