They're really not. Compared to many other popular hobbies gaming's yearly cost is really low. Things like audio, photography, cars, warhammer, travelling, winter sports each will have yearly costs that make gaming seem cheap as hell.

Football (soccer) is cheap, so is basketball, traveling can be done on a budget (backpacking, hitchhiking). Board games or card games (not the collectible or trade-able cash-grabbing variety) are also cheap.

There's many expensive hobbies, but also a ton of cheap ones.