I'm not going to defend the practice too hard, but I have been on the flip side a bit, and one MAJOR issue is longevity of not only the device itself, but the support for it too. So are industrial PLCs lagging behind desktop and cell phone technology? Sure. But they have to make them to LAST.
Now... how old is YOUR cell phone? 1 year? 2 years? Maybe 3? In my experience after 3 years, they no longer function. My PCs? Same thing. If I don't keep upgrading, I can't use them after a few years and then after 5, I can't even upgrade them any more, nothing works with the old OS after a while and the new OS won't load into the old HW.
PLCs used in machinery that is generating revenue for a manufacturing company cannot afford the luxury of only lasting a year, or 3, or 5... There are PLCs out there running machines for longer than many of you have been alive.
I just last month was asked if I had a way to modify a program on a PLC-3 that I installed in 1979! It is STILL running! I have a PC in my shop that I abandoned in 2012, but I realized my DOS based software to program that thing was on that hard drive and tried to fire it up. No joy, I had to remove the HDD and slave it to my laptop. Then I had to create a VM that would run DOS, then find a way to spit out RS-232...
But again, that was to CHANGE the programming. That PLC is still running, pushing on 40 years now.