It's become such a reliable tell that I'm genuinely curious why AI seems to think that cartoon images MUST look like they're drawn on old paper.

EDIT: The AI answer I got when asking Google was about what I'd expect, but this last bit I hadn't considered:

Sepia tones can help unify the color palette of an image, making it more visually cohesive and pleasing. This could be a factor in the AI's decision to use sepia, as it can contribute to a sense of order and harmony in the generated image.

So it uses it to try and make it more visually appealing to a wide audience, leading to it looking even more bland and uninspired.