Curmudgeon writes:

So why do horizontal condensate drain lines from AC units have a trap
bend?

(1) Keeps air from moving through it, which is inefficient in itself.

(2) Air flowing through the pipe deposits dust and fosters clogging through
microbial growth.

A "trap" in this case is a simple U bend somewhere in the run, usually at
the air handler. Not a more elaborate trap fixture like on a sink.