My guess is that stake weighted voting is one of the most sybil resistant methods of having a voting economy within a decentralized community where people can be anonymous.

That is to say, it can't be circumvented easily by just making a lot of accounts. It's a very pragmatic reason on top of incentivizing investors to buy more stake to expand their influence. Otherwise, if all votes were equal, more technically savvy individuals would just try to create thousands, maybe millions of accounts and siphon rewards that way.