Their job is to collect signals intelligence and execute cyber warfare operations. Not whatever you think it is.
"The National Security Agency/Central Security Service (NSA/CSS) leads the U.S. Government in cryptology that encompasses both signals intelligence (SIGINT) and information assurance (now referred to as cybersecurity) products and services, and enables computer network operations (CNO) in order to gain a decision advantage for the Nation and our allies under all circumstances."
https://www.washingtonpost.com/national-security/nsa-launche...
It's actually a rather fascinating incongruity, since we live in a world where "the enemy" is more likely than not to be using the same software systems that the NSA themselves are, and that therefore any exploitable flaws they find in enemy systems are pretty likely to be just as exploitable in their own. (And that similarly, disclosing the flaw in order to fix the issue in their own systems is very likely to result in "the enemy" fixing the flaw as well.)
A couple years ago the White House released a document explaining the process they use for deciding what vulnerabilities they keep secret: https://www.cnet.com/news/white-house-trump-administration-h... noting that "In the vast majority of cases, responsibly disclosing a newly discovered vulnerability is clearly in the national interest". Though from what we've seen in past leaks, it's pretty obvious they don't reach that conclusion for all vulnerabilities they find.