Instead of setting your devices to use a dns server of your choice it’s ignored by the apps
Some apps allow you to configure them, so now you’re configuring 200 apps on 20 devices rather than just one dhcp setting.
(Oh and OSs have generally broken hosts files)
What I'm suggesting isn't merely setting up the 'default' dns server. What I'm suggesting is 'cnaming' the name servers that apps attempt connecting to, to point elsewhere.
This general industry move will lead to more tls breaking proxies and more network interception. All because people don’t want to understand how a network and os work.
Making doh at the application later normal it moves the power towards the centralised advertising network and away from the individual. That the SV culture likes this is unsurprising.