Most of them are fake and don't really "pay" the coins they promised.
I have read numerous users saying they joined 100-200 airdrop and got about 20% of them only.
I have joined about 20-30 airdrops few months ago. I received about 5 coins, most of them less than 0.10usd.
I even made a post about it. Airdrops are not free, and are a waste of time:
https://bitcointalk.org/index.php?topic=3556844.020% would be fine with me too.
I fill these out in between loading screens or during lobbies when playing games, or during down time at work.
I have been careful about who I do a KYC with (still could be a risk) and I don't install any sketchy wallet apps.
I guess some people don't consider these to be worth their time, but when I have spare time I feel like I might as well. (I also day trade alts, mine alts, and put decent amounts of my own money into the less sketchy looking ico's, airdropping is just something a bit extra)
One thing to consider is where you are finding your airdrops. Some of the mailing lists I have tried signing up for have tons of BS looking projects, but Airdropsmob and Airdropalert are where I found all the ones that have actually paid out so far.