There are guitar and bass guitar amps that feature multiple frequency dedicated speakers, and even multiple amplifiers. These ones are usually aiming for higher fidelity, as for amplification of acoustic instruments, jazz guitars and the like.
As a bass player, I've tried and disliked some of these amps that featured high frequency horns because they tend to have uneven "spikey" frequency response that emphasizes the percussive clicks and noisy part of the sound from the strings, not the musical notes. The previous answer makes the most important point that the characteristic tone of a speaker in an amplifier is considered part of the instrument itself. Often "bad" speakers (in high fidelity terms) can have a great sound if you are looking for a certain tone, for instance a soft character when being over-driven, which is a trait of older, weaker speakers with paper cone edges.
On the other hand, if you want a pure tone, plugging directly into a quality high fidelity system works fine - I plug my bass guitar direct into my home computer recording rig which has a yamaha subwoofer and Alesis near field monitors and it sounds good. Many pop records have been recorded with bassists playing "direct" into the mixing board, and some guitarists will even play that way for a bright, clean sound. If the frequency response of a system is flat and realistic for music playback, then a good instrument is going to sound good playing through it.
So the reasons you see single speakers are more practical and cultural than technical.