Why is a Power supply of a computer still dangerous after disconnecting the power?

If you mean "disconnected' instead of "discovered" then it is because there is still power stored in the capacitors both in the power supply and on the motherboard. If you have any small lights on your computer, try unplugging the computer then pressing the power button. The lights should go on for a second and then go out. This action should discharge the capacitors.

This can act the same as static electricity stored in your body in that it can connect two wires and cause a short circuit and damage components.

For more information see the answer to the Related question shown below.