I don't follow this. Customer service is useful for customer retention in small firms, but customer service is also useful for building a brand in large firms.
The motivation for both is the same: selling as many vacation packages as possible.
Brands have inertia. Once your brand is big enough, you can get away with a shitty customer service and generally screwing customers over. If you aren't a total disaster of a company, then the worst that happens is that some of your customers book with another big brand the next year... but then customers who got screwed by that other brand will try their luck with you.
I'm really guessing with major travel agencies it works the same way as it seems to work with mobile operators in my country: you have three major networks to choose from, every one tries to screw you over as much as possible, but since everyone needs a smartphone, people just bounce around between the three in anger (or try to arbitrage deals). Viewed at scale, it's just an equilibrium.
The motivation for both is the same: selling as many vacation packages as possible.