Why are there no tanks in Fallout 3 / New Vegas?

Looking at the weaponry and vehicles used by the Enclave, I would guess that the creation of heavy-duty personal armour, along with an increase in the power of handheld weapons lead to a focus on mobility.

If you can wear the armour of a tank, and carry the weapon of a tank, why do it in the open inside something that can't manoeuvre nor be transported as easily as a person?

Thinking about it. You could make an even better armoured tank with even bigger guns. And presumably make it taller to increase its effective range. And give it legs so it could cross terrain steadily.

But that would be some kind of ridiculous giant death laser robot that wouldn't even get finished before the war ended...

The New Reno Arms shop stores in its basement an artillery cannon. This is an example of heavy weaponry. There might also be some tanks there. New Reno is on the West Coast, to the south of the expanse explored in Fallout New Vegas. You can reach this location in Fallout 2.

From this I deduce: the cannon is stored in the basement because the shop was previously a military base or outpost. Its underground because nuclear fallout and its consequences potentially led to erosion.

Fallout 3 and New Vegas happen a great many years after the war. Any tank found was probably dismantled for its artillery, its iron, etc. After the first hours of war the country was almost totally nuked. Tanks would be at the border with Canada / Alaska, not inland. Otherwise they might be underground, hidden in remote bases.

One must also take into account the Lore divide, with Fallout 1, 2 and New Vegas on one side and Fallout 3 on the other side of the country.

The Real Answer: They probably just never modeled/designed the art asset.

The 'Excuse': All the tanks were at the front in the war with China. There was no reason to have them so far into the interior as Vegas and DC.