Imo, OSS advocates really really want open source contributions to be used as hiring signal, because that would mean more open source developers. That is what this all is about, primary. Wish to come back to "open source developers are superior, because reasons" ideology that was pushed for during open/close source wars. And close source developers take issue with it not just because of privilege, but also because you are claiming they are less capable due to their code not being OSS.
Majority of employed developers don't have regular open source contributions. Especially those who work in high pressure jobs that already take all they have - both in time and how much effort you can spend per day. If the company is trying at least little bit to reward contributions, the people who give the company more (or come in better rested) get rewarded more then those who work for company less. That drives employees incentives and behavior.
Yes, there are junior jobs available to people with two months of coding experience. That is how it should be and there is nothing wrong with company seeing your OSS code as one of way to prove you can do it. The moment it will be expected from everyone, guess what, people like me will have tons of advantages to homeless, former "big scholarship to go to university" student who had to drop out due to personal reasons.
Senior developer market is not flooded with people with large open source contributions through. It is just not the case. Why would open source on Github should be privileged over random portfolio in case of juniors? Or to open source contributions that are not on Github?
Lastly, I dont think large open source projects can handle influx of juniors using them to prove themselves - which is what would happen if companies would really require oss contributions for hiring. Oss projects have often hard time to stay on top of existing pull requests.