Because local ISPs are terrible at running DNS. They're slow, usually vulnerable, poorly scaled, and not very redundant. I've seen a lot of ISPs in the US have

Because local ISPs are terrible at running DNS. They're slow, usually vulnerable, poorly scaled, and not very redundant. I've seen a lot of ISPs in the US have "outages" that are merely "our DNS servers are down, so everyone thinks the internet is down".