U-joint on steering shaft: float or bolt down?

So, question: why not allow the splines on the steering box end of the U joint to float, rather than tightening down the set screw? Just like a drive shaft yolk slides on the transmission output shaft to account for flex/angle change in the drivetrain, why not float the U-joint on the steering box shaft splines
What if it floats completely out?

Also the drive shaft has splins designed for the purpose. Whereas the steering shaft does not.