For a lot of people, because they already have copper in the walls.

You are correct that 10GBASE-T really shouldn't be the default choice, fiber and DAC both have advantages over it. But compatibility is important, and there are a lot of situations where 10GBASE-T is just more convenient.

DAC cables can get expensive, and nobody knows what to buy when it comes to fiber, unless you're running entire spools of the stuff inside buildings... OM3/OM4/OM5? Single mode/Multi mode? LC/SC? Regular people don't know this stuff...

For connecting via say a Macbook Pro, there used to not be Thunderbolt 4 SFP+ interfaces. So, you were pretty limited to some ethernet SFP+ module that you hope would actually work.

Also personally, if you can get away with a copper DAC, I would rather use that instead of fiber because you don't need any special modules.

Access points, for example, expect PoE.