Except with 10/8 (or 172/12) everyone is using the same address space. How many network have a 10.0.0.0/24? What are the odds of a conflict for that?
But if you have an ULA FDxx:xxxx:xxxx/48 address space, what are the odds that all those x bits will be the same for any two sites? That's 40 bits 'entropy'. Much, much lower (notwithstanding folks doing DEADBEEF, BADCAFE, etc).
Sure, some network engineers will try and design v6 networks as though they're v4, but everyone else will just go and get a squillion addresses from the RIR that's unique to their org and then just use/announce that.
Nothing stops you getting a subnet allocated from your local RIR and announcing that on the internet, if you care about that sort of thing for your home network though. You just need a decent ISP.
I’ll just leave that there.
No, I can’t just choose to have a decent ISP. I live in the real world where there’s only one choice where I live, same as a rather large majority of people in the world.