Afraid it will invalidate warranty.
Leather and studs make it too hard to raise arm.
Refuses to wave to anyone whose bike is already paid for.
Afraid to let go of handlebars because they might vibrate off.
Rushing wind would blow scabs off new tattoos.
Angry because they just took out a second mortgage to pay luxury tax on new Harley.
Just discovered the fine print in the owner's manual and realized that HD is partially owned by those rice-burner manufacturers.
Can't tell if other riders are waving or just reaching to cover their ears like everyone else.
Remembers the last time a Harley rider waved back... he impaled his hand on a spiked helmet.
And the final reason Harley riders don't wave back:
They're jealous that after spending $30.000 they still don't own a V65!