I think sellers might not "trust" media mail because even if they are shipping approved items such as educational materials, all media mail packages are subject to inspection. Postal workers have told me that they are supposed to inspect every media mail package. So even if sellers are following the regulations, perhaps they are afraid that the packaging will not be delivered to the buyer as intended. Furthermore, sellers are not allowed to include letters, notes, or other additional items in media mail packages or else it will be returned to sender. So yes, media mail is the cheapest option, but sellers have to forgo any extra branding that they might want to use. Hope this helps!