The most profitable investments are on the seed and pre-sale stage. But they are very risky.
Airdrops allow to get tokens of new and prospective projects for free. That is why airdrops are not so risky.
During investment you spend money in new project, during airdrops you spend your own time in new project.
That is why many people prefer to participate in airdrops to get free tokens.
They risk only their time, but not money.