Why did you not realize that “this guy is worth it” until he already has one foot out the door?
If his only reason for being dissatisfied with his current job was the money, why didn’t he just go to his manager and say “hello, I was looking at salary surveys and other information about the job market, and reviewing all the ways that I have made money for you guys over the past year, and I think I deserve such-and-such more than I’m getting right now”? Why is he finding staying in the familiar environment of your company less attractive than job-hunting in his spare time, possibly even burning vacation time for interviews, and then going out to work with a bunch of strangers?
Not everyone knows their worth and some people avoid confrontation (you are confronting your boss in a way).
Some managers might overlook some people (not saying they are good/bad at managing if they do).
I had the experience of being underpaid (while I was still in school so it wasn't all about the money) but after I graduated I wanted to make at least close to what is going around here.
I had it pretty lucky that I could just waltz into the boss's office and say I need to make more.
It had been over a year and I had graduated in that time frame so I felt I had a bit of an argument and I am the guy who hates confrontation.
At that point it was pretty much only about the money as almost everything else I did like.
> Why did you not realize that “this guy is worth it” until he already has one foot out the door?
There are plenty of reasons this could happen. It's a mistake, and you should analyze the situation to see why it was made. But that doesn't imply you should let people walk out the door just because you made a mistake.
You assume that anyone who comes up with this thing implies a particular scenario, when it really doesn't. It could simply be that e.g. an employee got a cold call from some friends who recently got some funding and is toying with the idea of switching jobs, and the higher salary is the one thing that really makes him think of taking the offer.
If his only reason for being dissatisfied with his current job was the money, why didn’t he just go to his manager and say “hello, I was looking at salary surveys and other information about the job market, and reviewing all the ways that I have made money for you guys over the past year, and I think I deserve such-and-such more than I’m getting right now”? Why is he finding staying in the familiar environment of your company less attractive than job-hunting in his spare time, possibly even burning vacation time for interviews, and then going out to work with a bunch of strangers?