Is it possible to replace a shot-off valve under my sink with out cutting off main supply water line of all units. Why they didn't install a cut off valve in this fu** stupid builder.

I would NOT advise trying to replace a shut off valve without turning off the water to the whole house... not unless you want a huge water mess on your hands that is. You must cut the water off. Some builders don't do this as a cost cutting measure. The valves cost money, my dear, and they just didn't want to spend $5 extra on that feature. Go figure.

is it possible to replace a shot-off valve under my sink with out cutting off main supply water line of all units. why they didn't install a cut off valve in this fu** stupid builder.
No! While we could install a bank of shutoffs to each branch the cost of the valves, the extra pipe and labor involved make this not very cost effective. Back when I first started plumbing they didn't even put shutoffs under any of the fixtures. Shut the house down and change the angle stop out. Good luck, Tom