Not the only problem, with some engines the, about 3/16" inch, the threads are exposed to carbon built up in the combustion chamber with rock hard carbon, those threads aren't doing a thing to hold that plug on.
And what is holding the spark plug in? The friction between the base of the plug with the gasket to the surface of the head, that doesn't get any anti-seize on it, just like a painted surface on the thread themselves, and you cannot deny that dissimilar metals like this with a constant current flow to ground causes electrolysis.
Course, another problem is leaving them in there for 75K miles or so.
Another problem is the boots themselves, they bake on to the plugs. With spark plug wires, had to make my own tool, piece 5/8" flat steel with a U cut into the end of it bend to a 90* angle. Pulling on the boot is like a Chinese finger puzzle, harder you pull the tighter it gets. But getting this tool under the boot compresses it, so you can pull it out.
Can't do this on the Cruze, a blind boot, so they get plenty of dielectric grease. Not even sure if you can buy spare boots, looked this up, only come with the complete coil pack. And those springs to hang up in the shoulder of those boots leaving a nice large gap between that spring and the top electrode of the plug. There goes your spark.
How about well over 35 years of designing and working with ignition systems, but had to be on a very strict budget. Did you know the entire 8 cylinder HEI distributor had to be provided to GM for six bucks! That included all the machine work on the distributor it self, the cap with the coil on it, all the electronics on the inside, plus 8 silicone spark plug wires with the boots installed. Other modules had to be done for as cheap as 69 cents.
If you were even penny over budget, were in deep trouble. Then had to deal with OSHA, EPA, DOT, ERA, etc., and production problems.