I work at a company that pays on the low end for its industry and I have watched no fewer than 10 people (of ~50) leave in the last two years. When I asked each of them why they decided look for another job, they all said they were not making enough to survive/save long term and they needed more money. I have stuck around, despite the low pay, because it is otherwise the best workplace I've ever had and I like the owners, but I have also recently started looking because I need to make more money as well if I am to have a secure future.

Maybe people do start looking because they're unhappy, but it seems like a cop-out to say "nobody left because of money", if the reason for the unhappiness is that they're not making enough money.