Its all in Ksp values. pKsp for Cu(OH)2 is 18.6, pKsp for Mg(OH)2 is 10.7. You start with mostly neutral solution - so concentration of OH- is around 10-7. Ignoring shift of water autodissociation solubility of copper hydroxide is 3.6*10-7 - so pH doesn't change by much, it slightly goes up by just a few tenths of the unit. Even if it goes up to 8, that means [OH-] = 10-6, [OH-]2=10-12, so you would need 20 M Mg2+ to see any precipitation. It ain't gonna happen.