Reply #1: the reason is that "lie" implies intent to deceive, and it's very difficult to prove that.

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even when it's transparently obvious to the casual observer, technically there's rarely actual proof that a political falsehood is an actual lie.

that's why they stick to describing such statements as "misleading" or "deceptive" because those are properties only of the statement itself; whereas, "lie" is a property about a statement (that it is false) as well as the speaker (that they knew it was false and meant to tell a falsehood).

for similar reasons, when someone kills someone else, they describe it as "homicide" rather than "murder" because prior to a guilty verdict, it's often very difficult for the media to prove the intent part of "murder". the most they will do is use the term "alleged murder", which again sticks to the easily verified facts -- that someone was ACCUSED of murder, without taking a position on whether or not they actually DID commit murder.