Consider that the usecase for a personal desktop computer may be different from that of a smartphone or tablet. There's more to personal computing than a web browser.
Of course an easier to use third party packing system would make the desktop easier to use, but that's not where the pain points are. (Especially since most third party binary packaged software is just something you unpack and run.)
Even the most easy to use packaging system you could imagine wouldn't impact the Linux desktop market share measurably.
Historical reasons like it being an open platform that developers could target without the requirement of third party approval, among other strengths.
Linux doesn't have anything like that traditionally. You have to make a deb, rpm, whatever Gentoo uses, etc. And ./configure && make && make install is laughably unintuitive.
Also, Google have done some serious work to ensure at least some backwards compatibility (and providing a support library, that i fail to recall the exact name of, for backporting various functionality).
While the FOSS world seems to lean on "compile it" along with "lacking resources" as an excuse for not maintaining anything more than incidental compatibility across releases.
The market share of any PC desktop operating system has mostly to do with the type of office work that took place in the 90s, when computers took over.
That's slowly changing as the web is taking over, and Chromebooks, Android, Mac, and "regular" Linux desktops becomes a viable platform for sharing your work with others.