In Fallout 3, I see deathclaws and rad scorpions fighting each other all the time, as well as Outcasts and raiders in the mix.

In New Vegas, enemies/NPC stay in their own zones and hardly meet each other. Either that or they are allied with each other.

I miss seeing enemies fight each other in the wasteland when I played NV, whereas it happened all the time in F3. That is probably my one major gripe about NV.

On PC, spawning mods will fix that. The Endless Warfare mod lets you set the faction/creature types, frequency, and roaming distance for spawns...carnage will ensue

That was always neat when encountered in Fallout 1 or 2.