The startup costs are relatively high compared to most other hobbies.
Personally, I ski, paint, swim, and play tennis. In order to paint, you can buy very good quality supplies, though not the best, for a relatively small amount of money and the startup costs are small, a few average brushes-$75, a starter set of paint-$100, and a canvas or 2--overall it might run you $200 to get started with a well above average painting kit that will not limit you in any way. For swimming you need a few suits, some goggles, and a membership with a local swim team--overall only a few hundred. Tennis is the same, a $1-200 racquet, a few balls, a nice pair of sneakers and you can be out playing at any local park for under $300. However, to continue these hobbies it gets expensive, as do any hobbies which you actively purse beyond the beginning stages, training is expensive for everything.
In photography, the startup costs, at least in the dslr range can be quite high. $500 for a body, $500-1000 for 2 or 3 lenses, $200 for a flash, $100 for a tripod... the applications are so broad that in order to just try everything in order to decide what you want to do, you can easily rack up thousands in expenses. Then once you decide what you want to do, the high-end gear is extremely expensive, $1000-$10,000 for a lens, $1-8000 for a body. Throw in a few flashes, filters, and other accessories and you can easily use $20,000 worth of equipment to take a single picture.
Skiing is currently my only hobby in which I shell out for top of the line gear, but even so, a pair of plug boots with custom lining and shell fit will only run you $800-1000, skis will run you about $600-1000 per pair, tuning supplies that give execellent performance are about $500, and a top-of-the-line clothing setup will only cost you $1-2000. I have top-of-the-line everything and have only spent about $4000, the cost of a single 300mm telephoto lens, on sale.
Sure, you could get a great P&S for about $500 and take good pictures, but the flexibility is just not there.
just my $.02