It isn't. The baseline profile is limited to 9 megapixels, the advanced profile is limited to 36 megapixels, but you can also have a valid avif file that doesn't conform to any profile and that is larger. For software decoders that will not be a problem. Hardware decoders in consumer devices will likely most typically be limited to the baseline profile: only 4:2:0, at most 10-bit, and at most 9 megapixels (e.g. 4K). Tiling can be used to combine several independent bitstreams in a single AVIF; there might be visible seams at the tile edges though.
It's a limitation that makes a lot of sense for a video codec — video requires hardware decoding to keep battery consumption sane, and hardware decoding requires a limit to the frame size since hw implementations need bounded SRAM buffers (and the cost in gate count is more or less proportional to the max frame size).
It's a limitation that makes a lot of sense for a video codec — video requires hardware decoding to keep battery consumption sane, and hardware decoding requires a limit to the frame size since hw implementations need bounded SRAM buffers (and the cost in gate count is more or less proportional to the max frame size).