Plenty of people are concerned about any US based company having access to their DNS logs; even with the pro-privacy agreement CloudFlare have in place with Mozilla, the US government has past form of doing bulk data collection from US based companies.
Your ISP's DNS may well be covered by relatively strong data protection laws (versus those in the US) and don't lead to a foreign government gaining access to all your browsing history, even if in principle they could be MitM'd.
You manually opt in to 8.8.8.8. You may be using something with stronger legal protections than Cloudflare and will not be aware you need to opt out of this. The strength of encryption on the network is irrelevant if you don't trust the recipient.
why isn't DOH possibly more protecting of your privacy than these other services?