of course, a signature can only be 74 bytes apparently so i don't know why they would do that since OP_RETURN is 80 bytes.
Again, it is not a consensus rule, but only a standardness rule:
I came to agree with Gavin about whitelisting when I realized how quickly new transaction types can be added.
why not make it easier on everyone and just allow say, 64 or 128 bytes of random data in a transaction?
That's already possible. <pubkey> OP_CHECKSIG. <pubkey> can be 33 to 120 bytes.
I also support a third transaction type for timestamp hash sized arbitrary data. There's no point not having one since you can already do it anyway. It would tell nodes they don't need to bother to index it.
And the same is true for signatures: they can be much bigger. It is non-standard, but valid. And you can always combine OP_CHECKSIG with OP_NOT to make it spendable, or do other tricks, like 1-of-3 multisig, where only one signature has to be valid.
Also, this post alone, written by Satoshi, can show you exactly, why OP_RETURN was introduced in the first place: because there are other ways to do the same thing, and those other ways can be more abusive, than having a standardized way to ignore a given output.