This is me. One that sticks in my mind is Westside Story. What kind of credible gang struts, clicks fingers and bursts into song?
I think West Side Story marks the point where (big generalisation coming up!) musicals became pretentious and dull, with the cringiness dialled up to 11. I'm totally allergic to the Bernstein/Sondheim/Lloyd-Webber genre, including all their present day imitators. I do like some later stuff - Barbra Streisand's great, and I still watch the Elvis movies I loved as a little kid - but there are really only two kinds of movie musicals that I like, and they are all pre-1960. The first is the lavish, Busby Berkeley, Astaire-Rogers type of extravaganza - you could put Singin' In The Rain in that category and it's probably the best of them all. Then there's the kind which is really just a star vehicle for an extraordinary singer - Bing Crosby, Judy Garland, Al Jolson, Richard Tauber, Doris Day - and these are the films I obsessed over when I was younger. I guess you could put Marilyn Monroe's musicals or those of Marlene Dietrich in the "star vehicle" category, except that they weren't really primarily singers, so I mostly watch those kind of films to ogle the costumes and interiors.
Back in the early 90s Channel 4 used to show old movie musicals at 2pm on weekdays. I'd go back to school and leave the video set to record, then come back home to some top-quality escapist cinema. Remember the Tauber Pagliacci? How I wish I still had that VHS tape. It's totally unavailable everywhere now. It's what started me off on opera. Even though the score is a bit butchered, I think it would have been a good introduction to both opera and musicals for the kind of people who think it's so artificial when people suddenly start singing - both because Richard Tauber's Canio is a revelation (who knew he could act?) and because the play-within-a-play device allows a kind of nuanced exploration of the relationship between artifice and verisimilitude. (Very topical talking point when the opera was written.) I think I'm making it sound boring but it really isn't. I took the kids to see an am-dram Cav/Pag just before lockdown and they all loved it. The 11 year old was a bit upset by the ending and is now not sure about clowns, but she's asking to see the German film of Bluebeard's Castle again, so I guess the darker subjects in opera haven't turned her off permanently.
I love opera too, at least until it started getting all avant-garde and tuneless. A naked/Nazi-costumed eurotrash production of Strauss' Salomé is up there with Chess and Miss Saigon as the cringiest thing I've seen on stage. I do always like a good trad production of a tuneful opera, though. I'm fussy about singing but I don't expect much from live performances so I'm never disappointed. I think old movie adaptations of operas, as well as lighter musicals featuring the opera stars of the day, are insanely underrated. Anyone who's seen Louise with George Thill will know what I mean - I dare anyone to see André Pernet as the Father and then say that singing actors aren't believable! (I really believed he was going to throw Grace Moore out of the window at one point. :) )
I do think that YANBU and that people who "don't like musicals" were probably turned off by particular films or shows that were underachieving (like the Wicked with canned music mentioned upthread) or just part of a musical subgenre they don't happen to like. I'm not going to force musicals down anyone's throat, but there are vastly more of them than 99% of us have ever heard of, and you're bound to like at least one if you have the patience to seek them out. :)