As the Amiga was sort of high performance serious computing device, it was assumed that it is being used with an analog RGB color monitor or in high-res monochrome on standard video monitors and TVs, instead of inferior quality color composite signal on video monitors or TVs.
Since the A2000 already has three 4-bit DACs for converting the RGB to analog, it would have been more expensive to design in the colour composite video encoding circuitry and add the parts cost and circuit board area, while many would never use the colour composite output.
The monochrome output is easy, just add a weighted sum of the analog RGB outputs and the composite sync together as the luma output.
If you wanted to have a color composite output, it was possible with the addtion of an external adapter. Just like it was done with A500 which was often bundled with A520 adapter, to allow color composite output and RF modulator.
The point might be that the monochrome composite output is very sharp as it does not have colour information, i.e. no colourburst means no display tries to decode the colors, no colour carrier means it does not have colour pixel artifacts and can utilize higher bandwidth. So effecively composite capable monitors switch off their colour decoding and allow for higher fidelity monochrome video.