Look down on your keyboard. Look at all the keys. Now look at the spacebar. It’s not labelled. Why? Even the enter and shift and all those other keys are labelled. Not the spacebar. Why? Is there some reason why it’s not labelled or did they just get lazy and decide not to.
On a side note, just for the sake of fairness, I took out a sharpie and wrote “Space Bar” on mine.
Well, the character it reperesents is invisible, a non-character, a space
Shift and enter sort of reperesent actions, so require labeling (enter also reperesents characters - ‘carriage-return’ and ‘line-feed’)
But it is labeled. It has a space on it.
Also - in the days of typewriters - shift and return were not keys, they were levers, so when they ‘became’ keys they needed names.
IMHO, it’s hard to mix up the space bar with any other key.
It’s a religious mandate. That’s why you often see it written as Sp****ar, as it would be sacreligious to spell out it’s glorious name in an otherwise trivial setting.
Kind of like pl*co in the fishkeeping community? 
And it’s kinda hard to mix up the enter and any other key, but it’s labeled.
But why is it so doggone huge?
Naturally only one key can be unlabelled, which other one would you choose instead? I guess I don’t understand what it is you find so mysterious about this.
Why would you wonder about this is my question. Why not ask what the tilde (~) is suppose to be?
So it can be reached by one’s thumb when one’s hand is ANYWHERE over the keyboard. It is, after all, the most used button (I think).
Here you go: The History of the Tilde.
Au contraire! Backspace is certainly used more (at least by me)!
The mind bobbles at what other items in your home you’ve labelled…
Viagra.
Oh, yeah, and the thumb thing, too.
Whichkeyisthespacebar?
Maybe I’m crazy but I swear I’ve seen keyboards with the spacebar labelled “SPACE”.
And all this time I thought it was the “any” key…