Because it means making choices, breaking assumptions, etc.. They have made it user-customizable so they don't have to go through all that.

FWIW, the current de-facto standard is set by xterm. Here is a relevant excerpt of its source code:

    ! Disclaimer: there are no standard colors used in terminal emulation.
    !
    ! The choice for color4 and color12 is a tradeoff between contrast, depending
    ! on whether they are used for text or backgrounds.  Note that either color4 or
    ! color12 would be used for text, while only color4 would be used for a
    ! background.  These are treated specially, since the luminosity of blue is
    ! only about half that of red/green, and is typically not accounted for in the
    ! RGB scheme.
    !
    ! Blue text on a black background should be readable.
    ! Blue backgrounds should not be "too" bright.
    !
    ! Originally color4/color12 were set to the names blue3/blue
    !*VT100*color4: blue3
    !*VT100*color12: blue
    !
    ! They are from rgb.txt respectively:
    !  0   0 205  blue3
    !  0   0 255  blue
    ! However, blue3 is not readable on a black background.
    !
    ! Another choice was from the Debian settings:
    !*VT100*color4: DodgerBlue1
    !*VT100*color12: SteelBlue1
    !
    ! From rgb.txt:
    ! 30 144 255  DodgerBlue1
    ! 99 184 255  SteelBlue1
    !
    ! Some users object to this choice because the background (color4) is brighter
    ! than they are accustomed.  Others point out that the different weights for
    ! the red/green components make it appear to be not really blue.  Finally, it
    ! provides poor contrast against color13 and color14.
    !
    ! The current choice uses equal weights for red/green (effectively adding a
    ! gray to the result).  It is brighter than the original choice, and provides
    ! more contrast between color12 and color13, color14 than SteelBlue1 did.
    ! Contrast of color4 against black is slightly improved over the original.
    !
    ! Some refinement is certainly possible (you are welcome to try) -TD

Make that what you will :-).