So the downvotes on your perfectly legitimate comment are absurd, but here we are.
To get technical, the problem that you're having isn't that interacting with mailing lists is cumbersome, but rather, interacting with mailing list archives is cumbersome. You're not wrong. Web interfaces for mailing list archives haven't really changed in yonks, and there's little incentive to do so. The only people who use the archives are the people who explicitly do not interact with the list itself.
The people who actually use mailing lists, are members of the list itself, and receive the messages in their mailbox. From that point onwards, their interaction with the mailing list is only as cumbersome as their Mail User Agent (email client, MUA). Using a decent MUA that supports threading is step one to having an enjoyable experience with a mailing list.
Unfortunately, in 2021, we've regressed from a world where we have depth of options where it comes to MUAs, to where we are now where the vast majority use webmail solutions exclusively. Webmail has definitely improved since the 90s and 00s, but unfortunately not every feature has come along for the ride, making that also tricky.
As far as the barriers to entry go, thats subjective. Subscribing to a subreddit requires creating a reddit account. Subscribing to most forums requires signing up with an email address, password, possibly username as well, along with other biographical details (optional in some cases), and validating your email is real. Subscribing to a mailing list requires signing up with an email address and validating your email is real.
Anything you're not used to using, is always going to feel more cumbersome. If "happy mailman day" doesn't mean anything to you, you probably haven't had much experience with mailing lists (or have, but much more recent).