I mean, it's less about Robbie not being nominated and more about a Best Film contender of its particular subject matter getting a nod for Leading Actor while getting nothing for Leading Actress AND no nod for the woman that directed it.
At the same time, it's been pointed out that nobody seems to be talking about America Ferrera's nomination in Best Supporting Actress for the same film, but the general outrage is (IMO) less about the actual quality of the performances in Barbie¹, and more about Irony being served to us in a quantity that we weren't even certain existed anymore.
It could be said that if Gosling's performance was worthy of a nomination, it should stand to reason that Robbie's should also be worthy of same, though that's not how the Oscars work.
Indeed, what this could suggest in some senses is a greater depth of performances by actresses² this year; a condemnation of the collective male performances, suggesting that it's not that Gosling's performance was better than Robbie's, but that it was better than most other performances by other men in film this year; that Robbie's exclusion is more a result of better roles for women, and of the sheer amount of brilliant performances being put out there by women.
Whether that's the truth of the matter is a discussion for another time.
Gerwig's exclusion from Best Director, however...well, that's egregious. She accomplished something monumental with this film.
At any rate, it's brilliant that Lily Gladstone has been nominated, and I hope she wins.
¹ performances that were good, to be sure.
² we won't get into "actress" as a term that seems to only exist in the 21st century as a means of having more award categories