Because Red Bull was treating them as equals. You don't have to look any further than the qualifying at Austria for evidence of that. They alternated weekends pulling tows because the drivers were equals. Horner congratulated Ricciardo for wins as much as any team principal has ever congratulated anyone.
Go back to the interview at the end of the season with Max and Dan on the couch and you'll see Dan was punching holes in walls from early on in the season, not after all the problems with the car.
Maybe he did feel he wasn't being treated fairly, and if he had reasons then ok, but there is no evidence he wasn't being treated fairly. Some anecdote about Horner saying they want to make Verstappen the youngest WDC is not cutting it, because I just watched the press conference from Friday where he compared Gasly to all the other great drivers that have come through the junior program and that they expect big things from him when he settles in. A team principal talks up their drivers, that's part of their job.
I think someone got in his ear and convinced him he wasn't being respected, but he absolutely was, and I also guarantee they didn't sabotage their expensive as fuck cars to make Verstappen look better on their way to an easy 3rd place finish last season. Nobody could have anticipated Bottas' mental collapse that allowed Verstappen to get to 4th in the championship.
If he had re-signed for that one year deal they offered him, he'd be driving in a car this weekend that had a chance to win this race, and also have the (slim) chance to get Bottas' seat this Summer if he impressed enough in this first half. He could have still switched to Renault this Summer, because Ocon, who would have had that seat, would be the other person Wolff would put in Bottas' seat, so Renault would still have jumped at the chance to have a driver from a top team.
I understand why people have a need to believe that Ricciardo was somehow sabotaged, but to those of us outside that bubble, it's just makes no sense looking at what actually happened.