I wouldn't say it's 99% but yes, there are many 'scam'-airdrops (well, can you really call an airdrop scam? They don't steal anything except your time - well, and promotion)
However there may be other reasons for not receiving an airdrop you participated in - for example some ICOs fail and at the moment many ICOs are getting extended or paused so the airdrop-distribution may just be delayed.