Closed-source wallets may charge a fee every time a customer makes a transaction, and this fee is so small that it goes unnoticed. Depending on the number of users of such a wallet and their overall activity, the revenue may fluctuate significantly. Moreover, given that users or other developers cannot inspect the codebase of a closed-source wallet, it enables wallet creators to conduct malicious activities that also go unnoticed; for example, they are collecting and selling user data with information about senders and receivers, their balances, and their location. In other words, in the case of closed-source "free" wallets, you are seen as a product, and you are sold as a product. On the other hand, the business model of open-source software is pretty straightforward and transparent: in most cases, it simply doesn't exist because all contributions to the codebase are absolutely voluntary. Some wallets, especially those providing CoinJoin and other mixing services, may collect a fee for facilitating transaction obfuscation, but this is the exception rather than the rule.