Yes I was forgetting that bass is an octave lower than written. Doh!
Anyway, the issue remains that reproducing low frequencies is more difficult (the low notes on a cheap piano can be an indistinct rumble for example), and as Bill says the note we hear is made up of the fundamental and harmonics. But wanting to have a Jamerson like tone is achieved by removing the harmonics.
A couple of personal observations:
Lots of ERB players have a bright sound using 'hi-fi' amplification.
Lots of ERB players put bands round the strings to stop the unplayed string ringing, often at the twelfth fret, meaning they can only go down to C# below bottom E anyway. The ERB is often used to play in the register of traditional basses up the dusty end, not for pumping out low C#.
Surely the musical interest is not playing the same notes as the keyboard anyway - whats the point of that. I played with keys for 20 years and on the few occasions I doubled it was an octave or two up, or if he was playing up an octave or two down, rather than copying note for note.