Well, that's the truth, You need the money to keep the going . And the options available for web development are numerous as opposed to mobile (Maybe not desktop). Web development has a low barrier of entry and also a ready userbase for both testing and using.
You can easily cook up an only HTML site and send a link and ask me to check it out and I won't think twice before doing so, as opposed to mobile which have to be download same goes for desktop. Not everyone wants a mobile or desktop app, both a mass majority will want a website, so there is quite some money in there. Demands needs supply.