You asked what the Proton users are that don't use custom ROMs. I replied people that simply want an alternative to Google, Apple or Microsoft services. That does answer your question, with the reasons also in there: anticompetitive practices and privacy concerns, among others.
I'm not sure what you want to hear from me. People don't all have the same values (or give the same weight to said values) so they don't all act the same. It's not a binary where you either care so you go all in or you don't care.
Some of Proton's users will use custom ROMs, others won't (like myself). Reasons for that differ from person to person. Some want to go all in on privacy and get rid of everything Google/Apple, they'll likely go custom ROM. Others just want Big Tech to not read their emails for advertisments or block your account because you've uploaded pictures of your children to your cloud storage. Some just don't like Big Tech's tendency to be anticompetitive and don't want to support it. Some want to use a service not from the US. Some like how Proton looks and feels etc etc. You can switch to Proton for all those reasons and not want/need a custom ROM. And yeah, some only look at what's free and won't use Proton.
To take myself as an example, I am one such user that doesn't have a custom ROM, but uses Proton. Why? I simply wanted to move away from, in my case, Microsoft's Outlook and Onedrive because I didn't like them being able to read my emails and use it for ads. That doesn't justify flashing my phone, which has little custom ROM support btw, with the potential of bricking it.
Then I misunderstood. If it's just about privacy advocates, sure, most Proton users are privacy advocates in some form or another, though there are still alternatives to Proton in that regard.