What makes a input GPIO pin so sensitive?
Most modern chips have CMOS structure so they are not driven by current in or out, but voltage at a pin. CMOS input stage is basically a pair of FET gates and FET gates can be mostly seen as very small capacitance. So it takes very little charge into very small capacitance to accumulate enough voltage to make the input change between logic 0 and 1. Static electricity is enough, or a finger touching the pin. Or leakage current from nearby traces, via conducting dirt such as flux residue or moisture. Or just stray capacitance.

Since the chip is not absolutely perfect itself, there will be leakage currents inside the chip into IO pin. Maybe nanoamps or femtoamps, but there are. There can be different paths, one to pull voltage towards VCC and another to pull voltage towards GND. And they are temperature specific. So it is possible (and has happened) that a floating pin has stable reading at some logic level when cold and stable reading at another logic level when hot so it only works at certain temperature (e.g. device boots only when cold, or only boots when warmed up first)