In my opinion this is often due to the steel structure design codes. They often allow a deflection of around 0.004*span which on a large span allows large deflections.
If the support forces are transferred from the piping department to the structural department often the piping experts don't have a look at what structures are engineered to support the piping. My own restrictions for the deflection of piping supports are much more strict than what the codes prescribe just to make sure the pipe stress analysis is still reliable.

BTW Another example: I've once seen a fixed support being calculated with a for steel structures allowable displacement in piping axial direction of around 20 mm.