Putting my two cents... social media hyper awareness is just too easy an answer.
See, it was not just the stars who changed making people notice the anacronism of their in aesthetic beauty patterns.
The production of Hollywood movies changed, and with that it created a conflict between Setting Presentation x Casting Character Presentation.
What I mean by that is: in old Hollywood, ALL elements of a movie were highly stylized, so the stars were not the only fantasy sold to the audience, but the experience of watching a movie was transporting the audience to a fantasy.
It was embraced by the public and the production team that they would see everything be the complete opposite of reality, so the combination of the design and story and the casting collectively reflected that.
A technicolor western was completely presented as clean and glamorous so Marilyn wouldn’t be out of place in their saloons...
Gone With the Wind is a highly emotional melodrama where every choice of color grading in the photography, settings and costumes reflected the emotionality of the story and characters.
The Wizard of Oz, Snow White and the Seven Dwarfs and Pinocchio were Fairy Tales borrowing deliberately from different art styles and historical periods to create their own unique world.
Movies in contemporary post war settings like an American in Paris and Lili deliberately adopted an aesthetic inspired by impressionist paintings.
Biblical and Sword and Sandal movies like The Ten Commandments and Ben Hur recreated luxurious classic paintings and illustrations to emphasize the epic escope of the story they were telling.
But after the 90s, something happened... in an attempt to bridge a gap between Old Hollywood Glamour, Cynical and Grim New Hollywood, the Blockbuster Era and the rise of the Indepedent Scene, searching extra prestige the Hollywood movies adopted a more grimm, unglamorous photorealism, to be taken seriously... but in contrast, the stars didn’t follow that realism.
If anything, they became even more unrealistically idealized in their beauty!
Notice how after the 2000s, both on TV and and Theatrical Film Releases, the Period set Movies Loose More Color, the Photography becomes more Subduded in Brown, Mute Yellows to Oranges, or Sandy Greys...
But with rare exceptions the red lipstick of the peasant woman is impecable, the hair is always perfect, rare are the people who are fat, and everyone has their teen shining white and intact.
Bear in mind, when early directors between the 60s to the 80s experimented with grounded and cynical realism in comedies, westerns, war movies or medieval period movies, they often casted actors accordingly with rough appearances to fit that stylistic aproach.
But Hollywood, in an attempt to attend every taste at the same time in the same movie even when it doesn’t make sense, forgot that lesson.
The Big Hollywood studios want to shoehorn in the same project, ugly photorealism with clean and glamorous movie star.
And those two elements are not mixing well, so it wouldn’t take long for people to be taken out of the fantasy, notice and point it out.
I know our first impulse is to blame the audience when "we can't have nice things anymore" but when we actually engage with the history and art of film production, we discover more the combination of production decisions and audience reaction that generate the conversation.
