Why are input MOSFETs in Laptop power supply always in pairs?
You're missing the fact that discrete MOSFETs generally have their substrates tied directly to their source terminals. This creates a "body diode" in parallel with the drain-source path, and this means that such a MOSFET can only block current in one direction.

Therefore, many applications use two identical MOSFETs back-to-back (sources and gates tied together) so that when they're off, they can block current in either direction.