The reason for this rule is both simple and straightforward: a New York court isn't going to let you just back out of the contract later on if you didn't realize that something was broken or missing, or couldn't be modified in the way you had always hoped. That is, not unless the seller actively engaged in deception, and/or concealed this fact beforehand.
In legalese, this doctrine is known as caveat emptor (literally, "buyer beware") (Kelley v. Larkin, 24 Misc.3d 1201(A) [Sup Ct N.Y. Co. 2009]), and, as a practical matter, the buyer's attempt to undo ("rescind") the contract will be dead on arrival unless the buyer can prove that he did not have the means to discover the truth by the exercise of ordinary intelligence (Joseph v. NRT Inc., supra).
Parenthetically, this is another reason why fraud claims are so difficult to prove under New York law.