Whining is a pitch problem before it is a behaviour problem
Adults describe whining as manipulative, which gives a young child more strategic credit than the evidence supports. What is happening is simpler and older.
The whine sits in a frequency range that is difficult to tune out. It evolved to be un-ignorable, in the same family as an infant's cry. A child does not choose it the way they choose a sentence; it arrives when a request has failed and the system escalates to a channel that historically works.
Why answering it teaches it
If a request in a normal voice is declined and the same request in a whine is granted, the lesson is learned in one trial. Not because the child is calculating, but because the reinforcement is immediate and unambiguous.
The usual advice — hold the line — is right but incomplete. The missing half is granting the normal-voice version more often. A child whose ordinary requests are routinely refused has every reason to escalate, and no amount of ignoring the whine changes that arithmetic.
The practical version
Name the channel, not the character. "I can't hear that voice, tell me in your normal one" is repeatable and does not require the child to accept a description of themselves as a whiner.