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".

Try namebench https://github.com/google/namebench , it will give you the fastest name servers for your case (location plus usage)

Many people don't have meaningful choices of ISPs, and some ISPs only run a couple resolver clusters far away from customers, are just plain bad at running them, or are playing DNS MITM games.

Because while I have a great and fast internet connection, my ISP's DNS is notoriously slow with constant outages.

ISPs in some countries are forced by the government to block certain websites, which they typically implement with domain name blocking.

Google's cache is more likely to already be populated with the answer than any ISP other than the huge ones.

My ISP's DNS inserts advertising into query results. Google's doesn't.