Why not? Other than "because they're old"?

Tiger and Snow Leopard in particular were very solid releases.

Heck, the aluminum Macbooks from that era are still the foundation of Apple's laptop design. And they didn't have the butterfly keyboard fiasco!

But this is a bit of a irrelevant distraction. Apple under Jobs wasn't loved for quality of hardware, it was loved for telling a better story of progress of personal computing. From the iMac "make it simpler by going back to basics, but future-looking basics" to "easier to manage, funner to use music players" through showing how smartphones and then tablets could be far more functional and usable than MS', Palm's, or Nokia's visions. The watch is the next best category-definer since then, and the iCloud cross-device stuff generally feels better-done than competitors still, but otherwise... refine, refine, refine, and slowly add more ads and upsells. Microsoft or anyone else could run that playbook, in a way that they never could match the Apple playbook from 1997 to 2011.

(One side question here is "are there new segments out there waiting to be invented?" which I don't know the answer to. But even so, "becoming just another upsell-pushing, ad-driven, software-subscription-service provider" wasn't a necessary path.)

Snow Leopard eventually became a solid release. At launch it had many bugs, including some that lost customer data.

It’s tempting to compare one’s memory of an old late-cycle OS, after all the UI changes have been accepted and the bugs squashed, to the day-1 release of a new OS today, when UI changes seem new and weird and there are tons of bugs they knowingly shipped to hit the launch date (just like with Snow Leopard). But it’s not really a fair comparison.