Why generic cover letters fail
Hiring managers read the first two sentences. A letter that opens with "I am writing to express my interest in the above position" tells them nothing and reads like it was sent to fifty companies, because it usually was.
A cover letter earns its place when it answers one question: why this person, for this job, right now. That means naming the problem the role exists to solve and pointing at the specific thing you have done that maps to it.