Joe,

Because a cylinder is under pressure during combustion. Holding a spark plug against the block and getting a spark tells you nothing really, other than that your ignition system could make spark under no compression situation. Doesn’t really help you in the real world. I have seen MANY motors create spark in open air, then fail under compression. Always test spark under compression.

EDIT: I just read your wording, you mentioned opening the gap for open air tests. Didn’t catch that the first time around sorry. Most guys I see hold the plug against the block and when the spark jumps the electrode gap they call it good, which it may not be.