How does the glass screen of smart phones not block the discharge of capacitors in capacitive touch screens?
Short answer: the glass is part of the capacitor.

Long answer:

A capacitor is at it's most basic level two conductors with an insulator in between them. Capacitance is everywhere, all objects in the world will have some level of capacitive coupling between them.

When a finger touches a capacitive touch screen a capacitor is formed between the finger and the electrodes in the touch screen. The human body and the ground plane of the device containing the touch screen are also capacitively coupled to the world in general. The result is that touching the screen increases the total capacitance between the electrode and the ground plane of the device.

This increase in capacitance is what the touchscreen detects.