Using just a browser is way too token intensive and slow. It would look for 401 errors then run the browser automation to login with the credentials and grab the token.
Did you clone the Reddit API from browser traffic and then turn it into a 100% API driven thing?
I'd imagine they'd be sniffing browser agents, plugins, cookies, etc. to fingerprint. Using JavaScript scroll position, browsing rate and patterns, etc.
Maybe their protections just aren't that sophisticated.
The application-layer stuff is harder. Each application can develop its own heuristics, and that's difficult to automate in a cross-cutting fashion.
Reddit doesn't do anything about that? That seems stupid.
Easy for you does not mean easy for everyone. My experience is that TLS fingerprinting paired with blocking specific user agents gets a variety of majority of bot traffic.
It's the same a basic online security: You can protect against script kiddies with basic hygiene. If the threat analysis is Mossad, then yeah, you're fucked.