Its because google wants to give the apps to kids which they can not do if the apps show porn.
Its should however not include user generated content that has to be searched on these apps. Else a browser would have to be removed too.
However, that is not my expertise, so I can't say for sure that it is completely ok for teenager to watch porn.
Nevertheless, for the sake of this conversation, let's say, it is indeed really harmful for underage people to watch porn. Then, why don't Google or Apple just make a toggle? Or, since you already need an account to use Google Play or iOS App Store, just filter out all the porn apps when the users are deemed to be too young to properly enjoy porn. The fact that it is dangerous for a 6-year-old to drive a car does not mean cars should be banned for the entire humanity.
On the other hand, if watching porn for young people is indeed that harmful (enough for justifying banning them altogether on the largest 2 mobile app distribution platform), the future of humanity is already doomed, because they can already open up Google and search "porn" and start watching them.
How is that relevant? The law in any relevant place says you are not allowed to show porn to kids. Google has to follow that. Harmful or not is completely irrelevant same with the fact that kinds can use a browser to search porn. No one cares. Everyone just does whatever is needed to not get sued so they all do the same and delete visible unrequested/non-searched porn because that would trigger adults to sue. Its not about right/wrong its just money/lawsuits/PR damage etc.