But I also know that some movies got both, big box office and Oscar for the Best Picture. Below are several latest examples:
“Forrest Gump” (1994) — $709.8 million(adjusted)
“Titanic” (1997) — $1.2 billion(adjusted)
“The Lord of the Rings: The Return of the King” (2003) — $556.6 million(adjusted)
These are the exceptions basically and not the norm. Joker has good chances to win an Oscar(Won't deny that), but the competition is composed of veteran actors from Hollywood(Leo, Brad, Martin, Robert etc) which is why their chances of winning are way higher.
Maybe their chances are higher, but not as much as to not try your luck betting on Joker with 13.00 odds. I mean if the multiplier was 1.1x or something, I wouldn't bet on Joker because I too think that Parasite and Once Upon a Time in Hollywood are great movies which deserve Oscar for the Best Picture. But with 13x I will definitely bet on Joker.
Regarding your calling high grossing movies winning Oscars exceptions, well, maybe let's take a closer look at the Best Picture winners from the last 20 years.
Gladiator - $460.5 million
A Beautiful Mind - $313 million
Chicago - $306.8 million
The Lord of the Rings: The Return of the King - $1.120 billion
Million Dollar Baby - $216.8 million
The Departed - $291.5 million
Slumdog Millionaire - $377.89 million
The King's Speech - $414.2 million
Argo - $232.3 million
Green Book - $323.5 million
As you can see, 10 out of 19 Best Picture winners grossed over $200 million in the box office. So, those cases aren't that exceptional, in fact.