They simply do not. From the casino’s perspective, these bonuses are an expense for marketing. These platforms hope to attract new people which will describe their experience with the brand to others. Additionally, some players stay and deposit real cash. With a long term perspective, these bonuses pay for themselves.
Actually, a lot depends on the habits of players from each specific country. For example, those from Western Europe spend more money on slots. They usually make a small deposit after playing with no deposit bonuses. Russian and Eastern European players typically play with promo codes and look for a profit without the risk.