GPL software is a box that must be kept open, so that everybody would be able to take from it.

If you pick the box and build an altered version of it, you must keep it open, you are legally prohibited from attaching a lid to it.

There's nothing about any expectations, let alone obligations, to put anything back into the original box. Usually it's not very easy (you must follow strict standards) or even impossible (see e.g. SQLite).

The same goes for Apple and Google's OSS browser. The source is there, but there is more or less no way to give back, and they certainly don't.

Well, license obliging the original author to take patches back would be weird one.

But Google could suck in any change to their own and make it better.

> The same goes for Apple and Google's OSS browser. The source is there, but there is more or less no way to give back, and they certainly don't.

That's a different problem that's a bit orthogonal to licensing and has more to do with project leadership. Like, you don't even need to have OSS license to allow users to contribute to project.