After 1-2-2, opener has indeed shown 6+ spades.

If responder has 2, likely the contract will be in spades.

But -- there is no hurry to raise with only a doubleton.

Better is to have the raise promise 3.  This makes it much easier for opener to evaluate slam prospects. If opener has, say, K98654, and partner can raise from 2 to 3 with, say Q2, obviously there is a big problem for slam.  Knowing the raise promises 3 (thus at least a 9-card fit) makes it much better for evaluation purposes.

Meanwhile, if responder has only 2, he can mark time with some other call. Usually 2NT.  He can later raise spades to show a "preference" (a doubleton) -- and opener will know it is only a 6-2 fit. This also helps the partnership sometimes reach 3NT instead of a 6-2 doomed 4-of-a-major.