or is there some underlying factor causing these services to all tighten their respective nooses simultaneously

I'm guessing, uhh, money.

I thought Reddit and Twitter were pretty clearly responses to the massive data grab by OpenAI that let them train their language models.

I assume there are other factors involved too, like Reddit trying to force everyone to use its app. But I don't think it's coincidence this happened not long after ChatGPT took off.

... Though now that you mention it, I wonder if there's a connection between Netflix stamping out password sharing and Youtube getting serious about ads.

As with all innovation that was started at behest of public (taxpayer) initiatives, companies are initially greeted with wide open frontiers - new horizons to exploit, extract, and dominate.

In the fledging stages of a startup, they will sing praises of openness and public good and the importance of community. However, once they have reached a certain point they pull the ladder up behind them, close the door, and claim they did it all based on their own ingenuity and merit.

We're it not for the effort of the public - through the creation of the internet, and the effort the average person has put into making content and value on the internet - these companies would not exists. Don't forget that. People; regular people, are what make the internet a place worth visiting and using. These companies are just extracting from the richness that we generate.