It's not. In general, people who say it is fall into one of three groups:
1. People who want to censor and/or surveil other people's devices and traffic. These people hate DoH because the entire point of it is to protect against them doing so, and by running over port 443, it's really difficult for them to block it. This is the group that people who say DoH would be a good thing if only it used a network-provided DoH server, or that they wish DoT would get used more instead of DoH, usually fall into.
2. People who have a workflow that it breaks. In most cases, there's some setting or workaround for that workflow that still lets you use DoH for most Internet queries, though.
3. People who say it's making the Internet more centralized or bad for privacy. These arguments are valid to say "don't everyone use Cloudflare as your DoH provider" but not to say "don't use DoH even with other providers", since there's no reason DoH servers have to be any more centralized than regular DNS servers.
1. People who want to censor and/or surveil other people's devices and traffic. These people hate DoH because the entire point of it is to protect against them doing so, and by running over port 443, it's really difficult for them to block it. This is the group that people who say DoH would be a good thing if only it used a network-provided DoH server, or that they wish DoT would get used more instead of DoH, usually fall into.
2. People who have a workflow that it breaks. In most cases, there's some setting or workaround for that workflow that still lets you use DoH for most Internet queries, though.
3. People who say it's making the Internet more centralized or bad for privacy. These arguments are valid to say "don't everyone use Cloudflare as your DoH provider" but not to say "don't use DoH even with other providers", since there's no reason DoH servers have to be any more centralized than regular DNS servers.