The Main Frame of yore was ... one of the 19 inch racks the computer was made out of (the main one, typically with the ALU and registers in it). And those racks were used because that was how phone systems were built (phone systems had a lot more repeated / regular structure than something like a computer, though of course everything has converged). The "frame" nomenclature came from the phone system too.

So "normal servers today" got the rack from the mainframe, the mainframe didn't adopt it because of how datacenters were built. All the familiar stuff (raised floors, chillers, backup power, fire suppression, controlled access doors and so on) come from the mainframe world.