Because it’s a rule, mostly driven by gambling. NFL doesn’t want to get sued by sportsbooks/bettors.
Per the NFL’s injury report policy, NFL teams are required to report any ailment which “may affect a player’s ability to play”.
“If after the Friday update and prior to the 90-minute meeting at which the club submits its list of inactive players, an injury or other event occurs that may affect a player’s ability to play, the club’s Public Relations Director is required to report that information immediately to the local media, The Associated Press, the televising network, the Public Relations department of the club’s game opponent and the NFL Communications Department.”
Bengals knew Burrow had an ailment that may affect his ability to play, thus they were required to disclose it.
You’re gonna get downvoted because we’re on their subreddit, but they should (and likely will) be fined.