It's a legacy of the era when IBM ruled the roost. All of the mainframe vendors were like that.

There was an office space in the middle of our data center at my first big IT job. Only Unisys people were allowed to enter, and crossing the threshold into that room, or opening a Unisys cabinet was a serious offense.

I'd be secretive too! That "mainframe" was a $25,000 Xeon server with a mainframe emulator running on it. They had these sweet contracts where they would lease the server based on performance -- MIPS/mo.

The IBM mainframes are big power boxes with custom asics for i/o and lots of mumbo jumbo to make it sound magical.