And millions of people decided to make a company with no customer service their primary phone provider because it is free. Free comes with tradeoffs.
It's not free. It's no cost, but it's not free.
Considering the costs involved in a project the size of Google Voice, providing customer service isn't going to change the economics enough to make it unprofitable.
It's not impossible. The people at Google choose not to have customer service in order to maximize profits, rather than do the right thing.
Google has enough money to provide customer service. Customer service isn't going to bankrupt Google.