Another reason is that tea’s best company is tea, meaning that it stores better when it is less exposed to different odors and to oxygen. Storing in bulk thus keeps tea much fresher than storing in single servings (such as with bagged tea).
Lastly, we also prefer loose leaf because it is a more environmentally friendly way to consume tea. There is little to no waste involved, and you can easily compost the spent leaves. Try steeping your tea in a reusable tea infuser, a single use (compostable!) paper filter, a french press, or the classic--a teapot!