at some point someone gonna pay for the datacenters, the wires and all these stuff needed for software distribution.
Try running your own site on your own servers and paying for the data traffic and then do the math.
30% is ok
Say app has 10MiB, 100,000 downloads is ~1TiB, 1 have a 1GiB/20GiB VPS with that monthly bandwidth which I pay $1/month.
So distributing 100,000 copies costs $1.
I'll give Google $1 for distribution of 100000 copies of my app. Fair?
Yeah, I'll give Google a bit more for payment processing, perhaps. But actual distribution costs nothing really, for most apps.
I remember that being a problem in the app store for windows phones. Lots of fake apps that used the VLC name and logo, for example.
not every developer is willing to learn and spend time configuring & managing a server. that's not the same skill set.
First we're talking about the free app for a good cause, so you're already providing an app for free, so you've assumed some sacrifice for others for whatever reason, and setting up a website for it may be comparatively negligible part of the whole cost to you in terms of time.
In my case I would have already had an infrastructure and knowledge how to host things, so it might take a few minutes to put some new files up on a machine used for other projects. Additional costs being pretty much 0.
And there may be benefits (even if I didn't have the knowledge and infra) like learning something new, or now having an infrastructure and knowledge that helps you save expenses or time in the future. So that would offset the costs. You can't just look at costs alone.
Google should be thankful in the first place that developers are making apps on their ecosystem (doubly so for open source apps), because without them, Android and the Google Play Store itself isn't worth much. It's their part of the deal to stay relevant.