Have you ever gone to a supermarket where there's a hotdog stall that offers free taste. That's also the same with these airdrops, they are attracting people all in all by giving such tokens for free.
That makes these airdrop people announce and tell others that they've found a good token which results them to advertise that project.
You are right at this point but smart investors will learn from experience. The experiemce says that airdrop is no more profitable so its not afford to spend money in airdrop. So what we expect from airdrop? Nothing. But as people who want to get free token, airdrop is also good game.