No one is saying people can't move out. All we're saying is that in most countries in the world, there is no stigma to not moving out. Some kids move out when they head out for college. Others move out when they get married and set up families. Others move stay together, and the grandparents help raise the kids.
All of these should be okay.
In the US, they're not. The reasons people are staying are economical, but the reasons most people want to move out are equally broken: image and social pressure. People living with parents in the US are perceived as losers.
COVID19 exasperates this problem. Since the only people moving back in with parents are doing so due to economic reasons, the messaging that sends becomes stronger. I don't mind more than half of 18-29 year olds living with parents. That's only occasionally a problem. I do mind more than half of 18-29 years old living with the ENTIRELY ARTIFICIAL STIGMA of moving back in with parents. That's where the majority of the harm comes from.
(No, I'm not arguing there aren't abusive / dysfunctional / etc. families. It's a common. It's just not the majority case.)
"Involved" isn't the same as "coerced," US stereotypes to the contrary.
To get things out-of-the-way: All the data I've seen suggests that couples in such countries DO actually appear to have stronger relationships and people in relationships DO appear to like each other more (at least a few years in).
General analysis:
US: Couples go into a relationship with the expectation of a Disney-style romance, with a soulmate, prince charming, and all that. Sometimes, that's even there up-front, but people change, and 10 or 20 years down the line, "soulmates" will usually be less compatible. At that point, people are hit with disappointment, conflict, and quite often, divorce. The expectation of potential divorce also gives less incentive to invest into the relationship too. That's especially true in times of crisis. If you're hit by a car, does your mate stick with you or move on? People need to hedge.
PI: Couples go into a relationship with the expectation of initially being different, but of needing to the work to make it work, since they're in it for life. They're not disappointed when reality doesn't look like Disney, and usually they work together to make things work out. Couples tend to grow together over time, rather than grow apart, which is a healthier dynamic. In crisis situations, couples can count on each other to have their backs, since if things go badly for your partner, they go badly for you and vice-versa. Plus, you'll be counting on them in 50 years, so if you're not there for them now...
Refs: Modern Romance by Aziz Ansari.
But there is a more general aspect that parents can objectively evaluate compatibility and quality of your partner in a way that you can't. Again, it's worth noting there's a scale here. Do you inform your parents? Get advice? Ask permission? They pick, with your permission? With your advice? Entirely arranged? It's a spectrum.
I was excited by this statistic. If this becomes commonplace, it will carry less stigma, and hopefully happen more often where appropriate.