Probably because the same equipment runs outside in other places. The
Dallas-Fort Worth system is elevated,

I'm pretty sure the original one was like a roller-coaster, at or below
ground level (see below) and diving under the roads. Only four terminals
then.

The old DFW system built by LTV used lots of small vehicles and tried
too hard to do too many things, e.g., goods trains to move baggage
between terminals, and was hard to adapt when they divided terminals
into landside and post-screening airside areas. Bits of the track are
still visible. It didn't help that LTV left the business so there was
nobody willing to maintain or upgrade it.

The new system is a train with a single circular route with all stops
airside. If you want to change terminals landside, there is an
ordinary rather slow bus.