Many reasons not do use the provided DNS. First, you don't want to give the ISP more information on your browsing habits than it can already gather otherwise. Second, in some countries, ISPs censor websites at the request of of the movie and music industries. Those are enough reasons to rely on a neutral DNS provider like Quad9 or your own DNS server.

When that service actively spies on you, then yes, that is stupid...

That’s… weirdly a thing.

Do you use your car’s built in navigation function — that you paid for — or do you plug your phone in and use its free Google Maps or Apple Maps to navigate?