Shows like The Simpsons that were only made for broadcast never had a film negative. It was likely mastered to analog tape.
In fact there's an episode of the Simpson's where Bart buys an Itchy & Scratchy animation cel and is disappointed when it's just a segment of Itchy's arm (or something like that).
EDIT: https://simpsonswiki.com/wiki/Itchy_%26_Scratchy_animation_c...
The cel had Scratchy's arm, and it was in the episode "Lady Bouvier's Lover" (S05 E21).
I ordered an animation cel from Original Animation Art (Starshine Group).
You had to mail them a check and a description of what you were looking for, and hope for the best.
The first one they sent me was a little bit better than Scratchy's arm, but not by much. I returned it and asked for something with the entire character in it.
They sent Daffy Duck nearly off the edge of the frame with a nasty scowl. Not fun to look at.
For the third try, I asked if I could please have something with the character smiling and in the center of the frame. And they sent a wonderful cel of Porky Pig from A Connecticut Rabbit in King Arthur's Court!
It looks very much like this one, but only Porky and not the background:
https://www.comic-mint.com/chuck-jones/a-connecticut-rabbit-...
That cel must have been from the same scene as mine, as some elements of it are identical, particularly his right hand holding the torch.
Many years later I bought the DVD of this movie, stepped through it and found the exact frame with my cel. Too cool!
Even though you need really great quality originals to make it work.
I believe the reason they were able to remaster lots of old Japanese anime OVAs in HD is because the animation was recorded to film first. I wouldn't be surprised if the Simpsons just used videotape instead.