Airdrops and bounties are only very small percentage of project's tokens and I think that even they know that Airdrops only help to be known, nothing more, they don't bring new investors, but they email you and that an advetisement for the founders. I think that advertisement is the reason, why they still create airdrops.
It's just their way now to earn telegram followers, airdrops somehow where unnoticeable unless they gained followers, it can help to say that their project seems impressive.