Why Descent 1 and 2 don't ship with rebirth? Instead of using Dosbox-

Gog usually sells the game "as is" that is without obvious enhancements unless made by the owners of the copyright or if there is literally no other way to make the thing run properly.
And even in the latter case is usually Direct3D or Glide wrappers for early 3D accellerated games rather than full sourceports. If you notice even Freespace 2 is sold as a purely retail copy with compatibility fixes rather than the source port version even if the latter is technically better, same with Doom and Quake.

In the specific case here, there's also the issue of which version(s) to ship. Rebirth's last full release was in August 2013. They've done some betas and intermittent snapshots since then, but even those have tapered off. At this point, the choice is build-your-own from a git snapshot (in which case, how do you pick what to package and how often to update), release something very old (and hope that it is at least as good as the retail build), or stick to the original Parallax retail build in DOSbox, which at least is a known quantity with well-understood features and limitations.

Some people may also not want to use Rebirth.

I use another Descent source port called D2X-XL. It's harder to set up than Rebirth, but looks better.

Note: The lava didn't look right on default settings, so I had to set some options higher which runs lighting calculations before each level loads, which takes several minutes. Just letting you know in case you want to try out D2X-XL (This issue may have been fixed in later updates,)