I tend to basically forget how to use a computer when people are watching. To prep for interviewing I also have to try to memorize a bunch of stuff about some relevant language because otherwise, without surrounding stuff to crib from, I'll straight-up forget basic shit about it, and that doesn't always stick. I'm talking like "what does method invocation look like in this language?" Under pressure it can and does happen for languages I've been working in 5 days a week for the last year. And I guaranfuckingtee if I write more than a few lines I'll end up using the wrong name for some standard library function I don't use daily—again, I drill these around interview time just in case, but it doesn't always stick. I'll also get really timid about using editor features, if I'm using a real computer and not whiteboarding, because I start second guessing every keystroke.
Further, most interviews I encounter don't do this and the ones that do tend to be all goddamn cagey about what to expect so I never know in advance, so I don't have a home whiteboard to spend hours and hours practicing on and if I did I wouldn't know which interviews need such prep and which don't so I'd probably end up not bothering, since it's just a few that do it.
I don't even have significant social anxiety or anything, and I have been told I come off as very confident and competent—unless the whiteboarding algo shit comes out, at which point there's a 50/50 chance I'll look like a total dunce (Nb. I can and do get The Actual Work done, no problem, once hired—that part's easy)
Since I only interview every few years and, again, this problem hasn't stopped me from landing jobs at about the same pay rate as the other ones that rake me over the coals (though not FAANG or near it) I've not spent the significant time it'd take to fix this. We'd be talking lots of hours to prep for FAANG tier, and probably a year. Don't care enough to do that. I did spend the time & effort to improve how I come off in more conversational parts, gradually, over the years, but that was relatively easy and doesn't rust as fast (since, you know, I don't get to practice interview-type coding on the job, but I do frequently have opportunities to practice relating to people).
[EDIT] I don't mean to be whiny or call the grapes sour—I simply haven't put the work in to (as I posted elsewhere in the thread) turn my efforts at leetcode from practice to performance. And again, for the roles I'm applying for I'm not losing any money by occasionally being rejected because they decided to give me a puzzle expecting a recursive & memoized (DP) solution rather than ask me whether I know WTF recursion and DP are and when they're usually useful, so the worst they do is waste a day of my time (honestly, why the hell are places so secretive about what's going to happen on interview day? Fucking Google's not, Jim Bob's Software Widget factory can afford to tell applicants the sorts of things they'll be talking about and the kinds of exercises that may come up)