That is very simplistic. There are a lot of other recurring costs that charities have - even the ones you mentioned have to pay rent at least. A women's shelter probably needs professional employees. Some charities have to operate planes and boats and those are very expensive. The most important thing is - recurring contributions help them plan long term and realistically gauge if they can take on more projects.
Also, calling a monthly donation a "subscription" is stupid. You didn't sign a contract - you can just stop giving the money whenever you want.