Beyond the obvious reasons mentioned, having it on a wall tends to prevent things from being set/put on top of it.
Connections in a breaker box can become loose/corroded and arc, causing heat which can potentially set other things on fire.
If buried under "things" it can be difficult to access when needed.
Codes not withstanding (which don't apply to most here anyway) it would work, but depending on the location, and potential use of the space it occupies, it may or may not be practical.
Wires should be in conduit if the box is on the floor, for their protection.
(For sure, don't put it under a mattress