Same reason human troops are better than ribots: they are cheap and make their own replacements.Why risk your troops? Have basilisks on swiveling platforms and replace your main forces with smaller numbers of animated armor. Combine the advantages.
On second thought, maybe you don't even need the basilisks anymore - you no longer need to feed or provide lodging for your troops. The logistical advantage alone is enormous - there are basically no supply lines to cut. Though the basilisks would probably help one animated armor go a lot further than a single soldier to make up for the costs of making them in the first place.