Seriously, i have search as many as i can and can't answer this.
It's way confusing when you visit plenty of certificate issuers sites and nobody truly says that google and apple REQUIRES me or NOT to BUY these expensive code signing certificates.
It maybe not a question for stackoverflow but i haven't found something related to it in the other forums but here.
For the platforms Android and iOS you need an code signing certificate. But a regular code signing certificate offered by a lot of commercial CA are not necessary (Android) or can't be used at all (iOS).
For Android you can just generate a self-signed certificate (uploading it to Google is optional). Just make sure to backup this certificate and private key as it is required for updating your app in PlayStore. For app publishing via Google PlayStore a Google Developer account is required ($25 USD one-time registration fee).
For iOS you need an Apple ID account and register for the Apple Developer account which costs you $99 USD per year (for certain persons/groups you can get it for free).