The problem with airdrops (even post TGE) is that there's no way to calculate the ROI and therefore impossible to tell if they're cost-effective. So why do projects keep doing them? My best guess is that they don't know what else to do with their tokens.

building @dau and @boostxyz to help app devs grow onchain with token incentives

Rare Fact. 
Many leave the project after TGE, and they sell all their allocation. Some people don't even know what problem project is solving in the space, but he or she does airdrop to get allocation and sell. The sad thing is that real users will get peanut allocation, and those that farm referral get fat allocation