Besides the obvious reasons:
- art materials are expensive. High quality art materials are VERY expensive.
- participation in exhibitions and conferences, and even rent a hall for a master class are costs money.
- self-education or learning from the master is also expensive.
Where to make money?
- Selling your artworks. Tons of sites like Etsy for it. Participation in exhibitions.
- commissions and other work to order. From portrays to pack of illustrations. Freelance sites, bulletin boards and letters to publishing houses.
- photostocks. In some of them you need to pass exams. Read the rules about publishing your work.
- selling of master-classes or art-courses. Independently, or with sponsors/art schools/art shops.
- PR in social media, if you have account and more than 500 followers. Paid posts. Also you can become an ambassador of some brand.
- art for videogames. Concept-art, illustrations, promo-art and so on.
- clothing design, print design and even your own merchandise stuff.