Maybe because floor is not stressed for chains.
Chains are essential for any heavy weight cargo.
If floors and tie downs are for straps only then Boeing should get out of the cargo heavy weight vehicle carriage.
Even 4 tie down straps rated at 5000 pounds each is not enough.
True vehicle may weigh less than this.
But if it breaks the straps that weight is increased many times.
Depending on the aircraft pitch, domino affect.