Why is Rijndael key length restricted?
The key schedule uses constants that differ between the key sizes. For arbitrary sized keys you would have to define an algorithm for deriving them. Each key size also uses a different number of rounds, for which you would have to do the same.

Also, what's the point? 256-bit keys are enough for all eternity. Using a longer keylength variant would likely just introduce a false sense of security.

If AES gets broken, it is IMO unlikely that such an attack would break a 256-bit key variant without also breaking an e.g. 512-bit variant with an equal number of rounds. The main exception would be if there was something wrong with the key schedule for one, like the weaknesses in the 256-bit variant, but in that case a less studied longer key variant would be even more of a risk.