Because they are "in-phase". In order to get 240v, you need two 120v Alternating Current lines that are 180° out of phase, that is, opposite phases. Only when one line is +120v and the other -120v will you see 240v between the wires.
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Your 2-wire system is actually connected to one "side" of the 3-wire system. In your utility's 3-wire system you have 2 hot wires that are attached to each end of a tranformer winding and the neutral is attached in the middle and grounded. Typically in the US this gives you 240v (or something similar) between hot leads and 120v from each hot lead to neutral or ground.
two hot one ground
There should be 2 or 3 wires. A hot, a neutral and sometimes a ground.
It is because the nec standard.
White wire is usually used as a neutral conductor and a 240v ac unit has no neutral. Each leg of the 240 volts is a "hot" leg However, very often a 12/2 or 14/2 NM cable is used to power small window ac units using 240v. In this case one of the conductors is white, but is not a neutral. It is good practice to mark the white wire with black tape so it is readily identified as a hot wire.