But anyway back to the discussion:
>The mass of one planet being able to simulate billions of planets is only possible if the simulation is rather poor
The mass of one solar system! And you don't need to simulate the system to a planck length. In fact the book specifically addresses that scenario - different simulations have different planck lengths and planck times, depending on the needs of the users of that simulation. For most users (certainly your average modern baseline human) that length might be on the orders of fractions of a mm, and some users might not require a physically meaningful interface at all!
>Everyone doesn't have to do it. Just one person out of billions does.
That's assuming that the culture doesn't have significant bans on von-neumann style self-replicating machines. If they allowed such devices of course you'd expect an expanding light cone of computronium around every significantly advanced culture but any sane long-lived culture would recognize that such machines are just about the only existential threat that a culture with those capabilities would face. That might significantly limit the 'explorer' class to those entities willing to do the equivalent of taking an ice pick to the frontal cortex in modern humans in order to undertake a multi-multi-millennia solitary journey into the void. Some fraction of those trips would fail, and the successes would take a non-trivial amount of time to launch another. Maybe those growth rates are smaller than we would suppose and the universe has only had stars capable of producing the elements of life for a little while on a universal scale, maybe those light cones just haven't reached this corner of space yet.
why do it?
Everyone doesn't have to do it. Just one person out of billions does. Think of the strange and perverse collection of behaviors we observe in a population of a few hundred million people online and speaking English. Expand that to billions of hyperintelligent minds and somebody is going to want to colonize the universe.