As biglain pointed out, there are people that can't upgrade. Someone who bought a Mac in 2008 is stuck with a release that doesn't get security updates backported. When you're an engineer it's easy to say "Well, that's a seven year old computer anyway. Just buy a new one." There are however people that either can't for financial reasons or see no point when it still works very well for them.
I know companies can't support old software forever though. I also know Apple has their hardware and software coupled fairly tightly so it may not be as simple as simply basing it on resource constraints the way Windows does. But client security is more important than ever and last I checked Apple won't commit to publishing support timelines which doesn't strike me as very fair if you're trying to make an informed decision when purchasing something. It would be nice if they were able to strike a balance between how they do things and Microsoft's noble but certainly expensive and painful commitment to backwards compatibility.
All that said, I have multiple Macs and I love them.
I know companies can't support old software forever though. I also know Apple has their hardware and software coupled fairly tightly so it may not be as simple as simply basing it on resource constraints the way Windows does. But client security is more important than ever and last I checked Apple won't commit to publishing support timelines which doesn't strike me as very fair if you're trying to make an informed decision when purchasing something. It would be nice if they were able to strike a balance between how they do things and Microsoft's noble but certainly expensive and painful commitment to backwards compatibility.
All that said, I have multiple Macs and I love them.