I agree in principle, but in practice, it is not always so simple. What starts as an uncomfortable feeling becomes a niggling doubt, becomes a suspicion, becomes a rumour, becomes a standing joke. There is seldom a moment when you catch someone absolutely red handed. The problem becomes a familiar background suspicion and the familiar is all too easily disregarded.
Reporting something with potentially career changing consequences for the reporter and reported requires something near to certainty and clear evidence.
Everyone knows something, but no one knows everything. Groupthink comes into play. Also, when something is everyone's responsibility, it is no one's particular responsibility, therefore it is always "someone else's responsibility".
A lot of the suspicion of Lucy Letby was based on correlations and coincidences. Just as a cricket captain can win the toss 10 times in a row (1/1028 chance) without being accused of cheating, a nurse can be on duty for 5 deaths in a row and it may simply be bad luck. Subsequence is not consequence, and many of us here will be familiar with the "margarine and serial killers" phenomenon: random correlation without causation.
Indeed, there have been cases of surgeons with a worse than average mortality rate, which has been traced to them being so good that they get a disproportionate number of high risk patients.
Add to this that Lucy Letby appeared to be a fairly normal young woman of her age and type. There are photos of her dressed up and socialising with colleagues and looking perfectly average. She was not a sinister creepy loner, she was "just Lucy".
All the weird diary entries etc. are known about now, but at the time that the murders were being committed, they were not. All that was certain was that Lucy Letby was present at the time of, or shortly before, a disproportionate number of infant deaths.
I remember decades ago working at a leisure centre where everyone "knew" that a particular colleague spent too much time alone with young children. This was the late 1970s when we were less aware of the grim reality of paedophilia. In fact, the individual actually used the name Peter Viles, which years later I realised sounded like "Paedophile(s)". Was it his real name? (Mods: I don't think there's enough here to identify the individual and he may well be dead by now anyway as he was many years older than me, and I'm now 60.)
No one thought it was their responsibility to report him, and we all bumbled along treating it as an uncomfortable joke. (In my own defence, I was a teenager with a very junior part time job, and lacking the life experience or confidence to be the one who raised the matter.)