Spotlights come under the "auxiliary lighting" rules, whereby they are only permitted to be used, when they can't dazzle other road users, ie. when main beams are on.
DRLs are designed for daytime use, so must extinguish at night, so to prevent dazzling to other road users.
DRLs also have a completely different light output compared to spotlights. DRLs diffuse the light quickly, so they're visible from a wide angle. Spotlight have tightly focused beams, so the light goes a long way down the road.
