Because it is not as bad as saying, “Don’t blame me for it.”
“It works on my computer” is one of those things that reminds me of how pedestrians behave as drivers and vice versa. The pedestrian feels that the driver should be careful and drive safely, but the same person, as a driver, hates all the jaywalking pedestrians. All programmers, at some point in time, have replied to a bug stating it works on their system, but it drives their team leaders (once programmers themselves) crazy when they hear them.
To the manager, this statement sounds insane. He thinks, “Why does it matter whether it works on your system? We are not talking about your system. We are talking about a customer who is unable to do their work because the application doesn’t work as it is supposed to? Why are you being so callous?”
The developer, simultaneously, thinks, “There shouldn’t be any problem with the application. After all, I tested it a million different ways. It passed testing and we didn’t have any problems so far. Maybe the person at the customer site is using an unsupported environment. Maybe they are missing a necessary component or turned off something for security reasons.”
Maybe this time, it is different, but probably not.
Yes, the developer knows it is not his fault. But saying that makes the developer sound too defensive. Better to use an actual fact, i.e., the application has been known to work. It is not a problem with the code he wrote. Inside, the developer also knows that he will have to fix the problem despite this statement. But he wants to win this mini-battle to maintain his credentials.
So, manager, when you hear a developer say, “It works on my system“, don’t burst into flames. Just say without any sarcasm, “OK, it is good to know that the application is working here. [significant pause] Now, can you call the customer and get it working on their system?“