If you run a mail server it's good to have a local DNS cache, e. g. Unbound. And if you have Unbound adding an upstream is not that necessary - it can talk to root servers directly or use hoster's DNS as upstream. I rarely use services like Quad9 but IMHO the main use case is for home PC / notebooks: e. g. if an ISP neglected it's own DNS (and it doesn't work well) or if an ISP spoof NXDOMAIN to return ads; another use case is free Wi-Fi where DNS can also be misconfigured / unreliable.
For an ISP it's relatively easy to provide a DNS server which will be fast and reliable (and your ISP's DNS is close to you than some 3rd party DNS) if that's not the case they probably just don't care.
If you run a mail server it's good to have a local DNS cache, e. g. Unbound. And if you have Unbound adding an upstream is not that necessary - it can talk to root servers directly or use hoster's DNS as upstream. I rarely use services like Quad9 but IMHO the main use case is for home PC / notebooks: e. g. if an ISP neglected it's own DNS (and it doesn't work well) or if an ISP spoof NXDOMAIN to return ads; another use case is free Wi-Fi where DNS can also be misconfigured / unreliable.
For an ISP it's relatively easy to provide a DNS server which will be fast and reliable (and your ISP's DNS is close to you than some 3rd party DNS) if that's not the case they probably just don't care.