Because there is no reason for them to be evenly separated. You're starting with the assumption that they SHOULD be evenly separated, and looking for a reason why they end up being something other than they should be. Your assumption is faulty. And so your question is meaningless.
they arent
The Asteroid Belt
compound
They were never joined to begin with.
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the 4 inner planets are rock planets and the 4 outter planets are gas planets separated by the asteroid belt
an Asteroid field between mars and Jupiter
If all planets are evenly spaced and there are 2,500 planets, they would be about 40 light years apart in the Milky Way.
The are separated by the asteroid belt, between Mars and Jupiter.
all planets arent the same color because they are all made up of different things
Mars and Jupiter.
Salad can be tossed to distribute its individual components fairly evenly throughout the total volume. However, it is not a homogenous mixture, as the separate components can still be distinguished and separated.