The three modalities differ first in what the output physically is. A text model produces a sequence of discrete tokens, one after another, where each token is drawn from a fixed vocabulary. An image model produces a single rectangular grid of pixel values, all of it settled at once. A video model produces a sequence of such grids, and the grids are not independent — they have to describe the same scene at successive moments.
That difference in output space drives the difference in difficulty. Text has the most freedom per step because a wrong token can be outweighed by the tokens around it; a reader repairs a slightly odd word from context. An image has no such repair mechanism: every pixel is visible simultaneously, so a misplaced detail stays misplaced. Video is hardest because it inherits the image problem and adds a constraint — each frame must agree with the frames before and after it. The number of agreements that must hold grows with clip length, and each one is a chance to disagree.
So the ranking is not about which model is smarter. It is about how many simultaneous constraints the output must satisfy. Text satisfies constraints sequentially and loosely; images satisfy them all at once; video satisfies them all at once, repeatedly, and consistently across time.