Since in my experience this trips literally everyone up (though less for bass since it is an even octave) I would elaborate a little more for Rich and say the reason this is done is for convenience.
The bass guitar actually plays an octave lower than the bass clef. For convenience, so that we don’t need to stare at funky ledger lines all the time and guess what note they are, bass music is notated transposed up one octave so it (mostly) fits on the bass clef.
The piano plays that low too, but since it also plays the notes in the bass clef naturally, it can’t transpose and must use the ledger lines for the lower octaves.
But this brings up an important idiom that you will see repeated more than once. Music notation is nearly entirely about convenience. It seems really complicated at first (and, well, it is) but most of music notation is about conveniently and concisely representing music in a written way.
There are instruments that fit entirely in the treble clef but transpose their notation by a half step just so that music doesn’t have to have sharp, flat and natural signs all over the place. Other instruments do it so that variously keyed instruments in the same family will share note fingerings for a written piece. It’s kind of maddening until you realize it’s much better that way. What it DOES mean is that you can’t necessarily take a score for one instrument and use it directly on another, though.