What do you mean by float? If you mean that work units can move through other units when mining minerals, that is a conscious design decision that was made for performance reasons. Having upwards of 40 worker units having to constantly calculate paths to and from the command center without being allowed to move through each other or move through other units that might cross their path would have slowed the game down to a crawl.
Of course people figured this out and managed to exploit it for other purposes, but those were seen to be too minor to bother fixing.
The mechanics of spider mines is they "hit anything that touches the ground". There are several units they don't hit such as all workers, vultures themselves, archons etc... This is all because those units "float".
I think “floating” units also have a different movement behaviour. They move a lot more like flying units. Do they also stack and transit through each other? I don’t recall.
Of course people figured this out and managed to exploit it for other purposes, but those were seen to be too minor to bother fixing.