Probably because they don’t meet IRS standards for nonprofit status:

An organization may qualify for exemption from federal income tax under section 501©(3) if it is organized and operated exclusively for one or more of the following purposes. • Religious. • Charitable. • Scientific. • Testing for public safety. • Literary. • Educational. • Fostering national or international amateur sports competition (but only if none of its activities involve providing athletic facilities or equipment; however, see Amateur Athletic Organizations, later in this chapter). • The prevention of cruelty to children or animals.