Because, for one, lengthening your position via a longer stem has less of an impact on stable handling than going with a shorter stem. (For instance, my 52 Domane came with a 90mm stem. Going up to 100mm is almost unnoticeable in terms of handling changes; going down to an 80mm stem on my 54 Defy made handling twitchy, especially in the drops at high speed. Not dangerously so, but not the most comfortable feeling.)

Point being, you can raise the saddle, get a longer offset seatpost, and a longer stem if a bike is a little too small; if it's a little too big, you can go with a zero-offset post, sure, but the stem changes start to impact handling, and bringing the seat too far forward can place your knee in the wrong place over the pedal.

Like [MENTION=355467]Doc V[/MENTION], I have long legs and a shorter torso for my size, and though I have long arms, they're not long enough to compensate for my short torso when it comes to reach.