But why? Apple is providing the whole ecosystem, operating system, security, notarization etc - there are costs to be covered.

Of course there are costs incurred per install. Previously there were covered by a higher 30% fee on app costs. Now, with opening up the platform to alternative stores, the EU has pushed Apple to a per-install model.

So to me, this seems completely reasonable as a standard iPhone user.

I understand the security and performance of the platform costs more - that is why I use an iPhone not an Android.