Because they are fermions, not bosons. Which has to do with their spin, not their charge.

See http://en.wikipedia.org/wiki/Pauli_exclusion_principle for more.

Because that's just the way they are :-)

More precisely, no two electrons can ever occupy exactly the same quantum mechanical state. http://en.wikipedia.org/wiki/Pauli_exclusion

Why the thing that keeps them from occupying the same quantum mechanical state by holding them apart (like in neutron stars) is not called force (the fifth one)?

That's actually quite funny that we call 'fundamental forces' only the forces that we can't explain the source of.