Why Mongolia is so popular among overland tourists?
 

Every year, there's plenty of overland tourists driving east from Europe either with a regular car, a 4x4 or a motorbike. Most of them end up in Mongolia or beyond. And there's even a few dedicated events for driving to Mongolia - the most famous being Mongolia Rally.

So, I wonder why Mongolia is such a popular destination? Besides a few landmarks, historical buildings and museums around the Capital, the whole country is so empty that is just grasslands and nomad tents.

You just answered your own question... bier

Most people who go to Mongolia think it is the only place where there are no roads. And that's something a lot of them want as you have to go off road for a few days to get back on the road. Most of people just follow highways or major roads to MNG and once there you can still follow the main road to UB, but the thing is that the main road is not asphalted, so the sense of adventure feels bigger than anywhere else.

I don't agree with my own explanation doh, but I know that most people will think exactly what I wrote down.
We had way more fun in KZ or RUS riding off road than we had in MNG for example. And most people won't go to KZ, they'll just stick to the major M road in Russia until MNG for example. Some places just have a connotation and people just think it is like this or that even if it's not necessarily like that...

I'ts a human nature thing
 

It's like when you buy a pair of snakeskin shoes and wear them for the first time; then you notice how many other people are wearing similar shoes, but you never noticed up until then.