If my luggage didn't make a connection and I arrived at my final destination without it, I would think of it as "lost luggage." Would it matter to me if it was stuck in the connecting airport or sent elsewhere? No, it did not arrive with me, the airline lost it, even if temporarily. In lay person terms, the luggage is lost--the airline lost a child, yes.
Yes, I understand your point -- colloquially, one might say that the "child was lost".
Your mileage may vary, but when someone who works for a company has done something horribly wrong and I'm trying to clean it up, I often find it helpful to be careful with my language so that I can express precisely and correctly what it is that they did wrong.
This is mostly a matter of showing empathy, not being a stickler for technical correctness. When I talk to someone who can fix a horrible mistake, and I show that I understand precisely what went wrong and who's responsible without overreaching, I gain credibility and the person I'm talking to (who is after all a person, if perhaps one who has done something horrible) is more likely to believe that I understand them. If I can express that I know exactly how their system works and what part of it broke, it helps them realize that I'm seeing things from their point of view, and this can help bridge the gap between "my side" and "their side" to help them realize that we're trying to get the same thing done.
I've found empirically that practicing empathy with people makes them more likely to try harder to fix mistakes and gets better outcomes. It also saves a lot of time to be able to say "X happened", which is exactly what happened, and not waste anyone's time re-explaining the problem instead of fixing it.
But, again, this is just a thing that's worked for me in practice (as it happens, mostly with airlines). Concise, precise communication about what went wrong, who's responsible, and what I'd like to fix the problem has helped me get great outcomes dozens of times.
In my mind, "lost" implies that its location can't be figured out at that moment. While airlines certainly do lose luggage like that, many times the airline knows exactly where your bag is but due to operational reasons it didn't make the same plane.
Sometimes it goes on an earlier flight, as was the case with my bag last month flying IAH-LAX on United, a route where United has a dozen flights a day. If you're connecting, your first flight may have been delayed and they couldn't get the bag from one end of the airport to the other.
I'm not trying to say the situation doesn't suck but there is a certain logic, twisted it may be, to airline operations.
And I'm certainly not trying to defend United's actions in this instance.
