A lot of ISPs are using these addresses internally as part of a carrier grade NAT since there are not enough IPv4 addresses to go around. So they do give them out, they just don't route them except through their NAT system, and they wouldn't want to route them as any effort would break their system.
Of course some ISPs have more IPs free than others.
RFC6598 defines CGNAT IP space as 100.64.0.0/10 (https://datatracker.ietf.org/doc/html/rfc6598). RFC1918 (private IP space) is a different allocation.
That's a relatively recent RFC; there are ISPs which have been using CGNAT for longer than that, and they do use RFC1918 addresses for their CGNAT (for instance, I know of one which used something in the 10.x.x.x range for CGNAT, which works in practice since most domestic networks use something in the 192.168.x.x range).
I'd be surprised (but not too surprised honestly) if the other ones haven't cleaned up the skeletons in their routers yet.