[cporios]

A taxiway that goes under a runway would require large, long inclines and declines. An A320 is ~12m tall, and with a 5% incline/decline (which is very large) you'd require 480m of horizontal distance in incline/decline, which sounds like a lot. And some airplanes are two times as tall as the A320, and with some margins added, it's probably safe to guess that an underground passing would require airplanes to taxi hundreds of additional meters to pass through the tunnel.

Airplanes burn a lot of fuel when taxiing, and taxi very slowly. So it's likely an expense, time and space constraint.