PCs tend to be in use for a lot longer than any given i or Android device too - I built my current desktop in early 2009 (i7 Nehalem) and simply haven't had the need to replace it (over the course of 6 years I've added more RAM and replaced the graphics card once).
On the mobile/tablet front, since then I've had:
iPhone 3GS (given to a friend)
iPhone 4 (sold)
iPad 1 (sold)
Samsung Galaxy Ace (sold)
Lumia 520 (current phone)
And I suspect I've invested in far fewer mobile devices in that time than many on el Reg.
A slowdown in desktop CPU innovation has stymied the PC upgrade market (why would I switch from Nehalem to Skylake?) plus PC gaming being held back by the console gaming albatross means there's simply nothing coming down the pipe (except VR possibly) to prompt a new motherboard etc. The system I have does everything I need it to (video, graphics, audio, games) - perhaps the next generation of consoles will finally render it obsolete for gaming and then I'll upgrade - so what's that? 3-4 years from now on its 10 year anniversary?
Oh, and my *last* PC (a Prestonia-based dual-Xeon) is now 13 years old and still going strong...