And the GPL makes it all work - as no single gigacorp can just take the whole and legally run with it for their gain, like they could if it was say MIT or BSD licensed.

So you have direct competitors all contributing to a common project in harmony.

Well, GPL is good but I think this setup would still be a local optimum for gigacorps, were it MIT or so. They are using plenty of MIT libraries, e.g. Harfbuzz.

It would just simply not make sense for them to let other companies' improvements go out of the window, unless they can directly monetize it. So it doesn't apply to every project, but especially these low-lying ones would be safe even without any sensible license.