This is actually a very valid statement and sadly, it's valid for a lot of projects out there. They clearly don't do it to drop the price because they don't really care about that.
But mainly because they don't want to be traced or the address being flagged or something so they just move to fiat as soon as possible and live a good life afterwards.
It might be one of the whole bunch of reasons but remember about ton of ICO projects which need to pay expenses and need to sell their collected Ether.