They take a penalty, though.
LTACs suffer an equivalent range penalty though, so not a decisive concern.
The full comparison seems to be:
Range 21+ artillery can fire indirect and hits a round later, LTAC can't fire at all.
Range 18-20: artillery can fire indirect and hits a round later, LTAC fires with a +4 range penalty.
Range 14-17: artillery and LTAC can both fire direct with a +4 penalty. Artillery can also indirect fire with no flight time if there is no LOS.
Range 7-13: artillery direct fires with a +4 penalty and LTAC fires with a +2 range penalty. Artillery can also indirect fire with no flight time if there is no LOS.
Range 5-6: artillery can't fire, LTAC has a +0 penalty.
Range 4: artillery can't fire, LTAC has a +1 penalty.
Range 3: artillery can't fire, LTAC has a +2 penalty.
Range 2: artillery can't fire, LTAC has a +3 penalty.
Range 1: artillery can't fire, LTAC has a +4 penalty.
Range 0: artillery can't fire, LTAC has a +5 penalty.
Overall, I'd say the above favors the LTAC in tactical combat, except that artillery allows for firing cluster munitions which are kind of like a free (no +3 penalty) called shot[high]. With that it's more of a wash---the LTAC does more damage (particularly with fuel-air munitions) while the artillery is more precise, finicky (no L6-), and adaptable (multiboard fire).