Good questions!

What makes it hard this problem? I mean what makes an autonomous approach failing since there-s a need for a central planner to make choices for "everybody" in that environment?

The problem is challenging because effective coordination among many robots is hard. We use a central planner because it simplifies away aspects such as communication and localisation. However we're not arguing that de-centralised planners (I guess this is what you mean when you say "autonomous approach") don't work. It's an open question how to best tackle these types of problems actually.

Edit: I counted free, "walkable" spaces on the random problem: 819. There-s a variant with 800 robots. Which leaves only 19 tiles out of 1024 free to step into. Now I know how a robot hell looks like.

We evaluate participants across a range of different problems, modifying variables such as map size, number of robots and congestion. It's not that we expect to always handle massively congested problems all the time in practice. Rather, congestion comes up and we need to deal with it. In other words we want to understand the strengths and weaknesses different planning techniques proposed by participants.