ISPs not only have a monopoly on connectivity and bandwidth prices - which they severely abuse by the way, considering the actual operational costs - but they also have everything to gain on analyzing your traffic, DNS queries being one of them. They already have a bunch of personally identifiable information (PII) on you (full name, date of birth, banking information and, in some countries, even social security number). Linking that PII to your DNS requests (read: what websites you visit) and selling that to data brokers is a pretty low effort sweet deal. Long are the days gone when ISPs only provided Internet service.
Texts on the topic:
- https://mullvad.net/en/help/all-about-dns-servers-and-privacy
- https://labs.ripe.net/author/babak_farrokhi/is-your-isp-hijacking-your-dns-traffic/
- https://www.howtogeek.com/why-dns-betrays-your-privacy-and-how-dnscrypt-stops-it/
Videos on the topic: