Why can Nami have boots but not Cassiopeia

Ok so I get that Cassiopeia can't but boots because she has not feet or legs or anything but like Nami has essentially the same problem the Cassiopeia has but she can buy boots. Plus champions such as Hecarim and Urgot have more than one pair of legs so why can't they have more pairs of boots? You know other than the fact that might make them a bit absurdly fast and break the game balance. And I know that Cassiopeia gains movement speed to make up for that fact that she can't buy boots but why not give her a different ability instead? Like her basic attacks poison you or something like that.