3 things.
1st is selective bias. People who take the effort travel and spend the money to play in OTB tournaments are a subgroup of chess players who are more likely to study chess.
2ndly, online play is generaly taken more casually and has more distractions. You might be playing a 3min blitz while waiting for your Wife to finish getting ready or playing a game in the living room while your kids monopolize the TV with nickelodeon on..... you can ad a hundred different distractions.
3rd is 2D vs 3D vision. There is a transition needed between online 2D play and 3D play. I even remember one of the first post COVID OTB events Yasser and Alejandro even commented on the (relatively) sloppy play and players transitioning back from 2D to 3D.
In OTB chess games the players attack harder and defend smarter. It also helps having more time on your clock usually.
Some players will resort to unsportsmanlike conduct like making inappropriate eye contact and putting their oversized water bottles on the table to intimidate you.