Why logic low Inputs to a logic gate need to be grounded
Will it not have zero voltage when in contact with air ?

No, it will not. Imagine an isolated node (input of an AND gate). If some charge get trapped on that node it cannot go anywhere. The node will have some voltage and a charge. That charge can be enough to open a MOSFET (and most AND gates use MOSFETs) which will influence the behavior of the AND gate.

Leaving logic gate inputs open / floating is an extremely bad habit as that makes the input undefined. The input does not have to be grounded, it can also be connected to the supply voltage. The connection can also be made through a resistor as long as the charge is not trapped on the gate's input.

But what if I simply discharge the input and then let it float?

Won't work either because there are ALWAYS leakage currents and non-ideal isolators (like MOSFET gate oxide) which cannot prevent some charge to accumulate on the input. So after some time there will still be some unknown charge on that input giving it some unknown voltage. Who knows what can happen then? No one knows, so the input needs a connection.