
Because he didn't need to.
Trump was doing what Putin wanted him to... trying to destroy NATO from within.
Trump's own National Security Adviser said that Putin was kind of expecting Trump to withdraw from NATO in his second term
The President has sole power to make treaties, subject to approval by Congress, so I guess Trump could have unmade it but under the NATO charter withdrawals do not take effect until one year after written notification of the withdrawal is received, and in that year even Trump's supporters in Congress would have deserted him and he would be impeached and the withdrawal itself withdrawn. Putin may not have known that but Trump would.
My take on it is that on paper, Congress has some oversight authority on our Treaty obligations.
But as a practical matter, the chief executive is the one who cultivates the alliances, manages them, implements them.
I have zero confidence in Trump's ability to cultivate our traditional European alliances
I have high confidence trump would have used a second term to subvert NATO and undermine American credibility in the alliance.