Because Executive Club is a frequent flyer scheme, not a frequent buyer scheme. if you need to the tier points that badly get on the plane.

This is a very long shot but I also wonder whether there is some legal aspect to it. You could create an alibi by by claiming to be onboard a flight to XYZ, your frequent flyer history notes it, when in fact you weren't. If it was somewhere in the EU you wouldn't even have passport stamps.