Stress just doesn't seem plausible. It would mean that WC/Euro/Copa/UCL/etc final stage games would be flooded with OGs, even without Covid.
Another factor is the difference of team dynamics in Club vs International football. The first three you mentioned are all international tournaments (nations play each other) while UCL is club football.
Consider that national teams only play a handful of times over a season (generally 2x per international break), and then on to a big event like the Euros. And all the other times they play outside of a tournament (friendlies, tournament quals) their manager is trying to win and tweak the team at the same time. Not to mention, the selection pool for a national team is very limited compared to what even a mid-table club could assemble (Arsenal, looking at you :P ).
So, national teams playing this tournament are not only stressed, they are also less-acquainted with each other.
I guess you could argue for comparing Euros and Copa America but another thing to keep in mind is the variance of skill level per player in this tournament. I find it reasonable to claim the variance is less in Euros than in Copa (quick reason: Messi, Neymar, Aguero, Coutinho, Firmino, etc. all play in European clubs but are eligible for Copa).
This gap in skill in Copa makes OGs less likely. South American defenders don't always deal with Messi or Neymar but the European defenders and strikers could've played (with or against) each other in club competition. This makes close situations leading to OGs more likely.
It isn't. See other comments in this thread for the more likely reasons (changed interpretation of "last touch" etc).
My comment was mainly replying to parent comment about why the players are/could be under increased stressed in this tournament.