There's everything you need in LOVE to create a GUI, so there is no need to include a built-in one. LOVE focuses on being a mash-up of game making libraries for graphics, input, sound, physics, sockets, etc. These are standard technical things which many games need, while there is no standard for in-game GUI's. If LOVE included a default GUI library, there is a much bigger chance that many people wouldn't like it because it's more of a creative thing than a standard technical thing (not that much, but still moreso than graphics in general, sound, etc.). Plus, you can make a GUI library out of the already built-in libraries, while there is pretty much no way to code graphics/sound/input/efficient rigid body physics/sockets in Lua from scratch. Many people are actually making GUI libraries for LOVE. All of them are pretty incomplete at this point, though. Perhaps you can help one of them.
There are already complete libraries for a camera system and simplified networking, though, so I think that LOVE is on track to becoming as high-level as people ever want it to be.
But then again, I may be completely wrong about this. Perhaps it'd be simpler to build in bindings for guichan or something? Rigid body physics aren't exactly low level, either.