Why don't the electrons of opposite spins collide in an orbital?
In reality, electrons in a molecule are colliding all the time insofar as they are experiencing a force (the Coulomb force) due to the presence of other electrons, and are correlating with each other as a result. Orbital models like the Hartree-Fock mean field approximation mostly neglect these interactions, so the answer to your question is they don't because the model doesn't consider such dynamics. But many sophisticated methods for calculating the effects of this correlation exist. E.g. "Full Configuration Interaction" and "Density Functional Theory" are two such methods.

So keep in mind that orbital models are only approximate. It is fine for many purposes, but inadequate for many too. The true ground state wavefunction of a molecule cannot be represented by a single determinant, and therefore cannot be thought of strictly as a collection of electrons occupying orbitals.