Why doesn't the AppStore/PlayStore have a pay what you want feature for apps

I have two apps on the Appstore that are free to download and while I never have any intention of charging a fee for this app, it doesn't mean I wouldn't want to make money if someone WANTS to pay me. This type of feature is great when downloading music off a site like Bandcamp. Lots of small bands have their music listed as 'Name your price'. I've even downloaded music and then gone back later and paid them because I really liked it. Most users probably won't pay and that's fine but shouldn't the platforms enable users to spend MORE money if they want to?