Electrically speaking, that is not a short. Shorts are caused by excessive current, blow breakers and burn wires and houses down. I know what you meant. Just busting your chops LOL.By internal short i meant the plug arcs internally from the center conductor to the metal shell.
You can't look at a spark plug and tell if it is good or not. You need special equipment to test a spark plug. For $2 it is more expedient for a shop to replace a plug and see is that solves a spark issue rather than looking for a high tension spark plug tester. Some of us buffons running small engine shops may have some skills other than mowers. I serviced printing equipment for 40 years. Some of it multi million dollar systems with some single repair parts costing thousands of dollars. You better be able to troubleshoot logically or else look for another job
when aircraft were in for scheduled maint the plugs were removed, cleaned and tested. If they failed they wefe replaced. Some radial engines have 36 cylinders with two plugs per cylinder and 2 or 4 engines. With 2 engines that is 144 plugs and at close to $10 a plug aircraft owners don't want to drop an extra $1500 for plugs. Yes we tested and documented new plugs.