I would love to pay other artists to put their work into my book, but at the moment the only sensible thing to do is to illustrate everything myself and take 5 times as long to produce a book. It's a little depressing for sure, but that's the industry we're in. The only people who stick around are the ones who do it for the passion of the medium.
Though it feels like society at large has less money to spend on TTRPGs, it has never been more popular than right now. If we all keep making our work and putting out there for the people who peek their heads into our niche corner of the art world, it can only get better. At least that's what I have to tell myself to keep going.