If they were suffering as bad as you ask them why they didnt killed themselves. They will be confused by the question, because to most people suffering is working 16 hour shifts some days of the week. Not really worthy of suicide.
I disagree with the latter part. Many people become depressed and even suicidal because of their poor working conditions. Worker suicides typically don't get covered, though. But even if it doesn't make them suicidally depressed, it doesn't mean it's not suffering. This is a pointless kind of comparision using the same kind of logic behind 'my experience of pain means you should live through yours'.
More generally...
I think one useful thing that can be obtained from these experiences is that some problems, in similar circumstances, can be shown to be fixable. That doesn't mean that new problems won't come up, or the person will attain the same result, or even that the person shouldn't suicide. But many people who are suicidal are not fully committed to dying until they have tried everything. I also agree that when this is usually said, it's said due to a poor frame of reference or even contempt (e.g. for perceived weakness) rather than a genuine effort to help and empathise.
Most people who haven't been suicidal find it an alien way to think, which is kind of understandable. We are raised with core assumptions about our existence, and our survival instinct provides the rest. If you're busy and your pain is comparatively temporary each time you experience it, you're generally not going to spend time pondering why you should die - it will seem self-evident that dying is bad. The chance of happiness outweighs pain in that calculus.
If you're suicidal, you might even be able to remember thinking that way yourself - and if you can't, then maybe not being suicidal is similarly alien to you. Also, people feel compelled to say or do anything to stop a suicide, even if they otherwise leave the person suffering and in pain, so their responses are often even more poorly thought out than you'd expect.
Nonetheless, it's a really annoying thing to say. It's minimising and silencing.