The first computer speakers were a single small speaker built into the case. It was used to produce a beep or series of beeps to notify the user of things like errors, especially hardware related where displaying something on the screen might be impossible. Programs would call the beep command to notify the user of something.

This basic beep or not beep evolved into tonal beeps and introduced very rudimentary sound effects (much like that of the old atari games). These were still played through the integrated speaker.

Eventually the programs began to use more sophisticated sound effects and music. The desire to be able to play more than one sound at a time lead to the development of sound cards and the introduction of external speakers that would be capable of better sound than the single small integrated speaker.

With the expansion of computer use into full multi-media devices the speaker systems available for computers are equal with that you can get for a home entertainment system including full surround sound with bass and the works.

Because computer speakers are no different than any other kinds of speakers.

If you want to know the history of speakers, you could try here.

http://en.wikipedia.org/wiki/Loudspeaker#History