In team-based group start road racing, like TdF, a lot of people aren't really competing. They are top sportspeople by ability, but their job is to support the team star. They are often called in French "domestiques", servants.
I wonder if this contributes. Imagine you're a sport person, your job depends kn your performance, you are at the mercy of your team, and it's not even like you can win. So why not help yourself to some pills.
But then, as siblings say, I don't even know if cycling is worse than other sports.
I think the format plays a huge factor too but for different reasons. This format of racing is very dependent on aerodynamic advantages - to the point that even on the massive climbs the rider on the wheel still holds the edge to someone doing the work. On the flat stages the peloton is almost always going to catch a breakaway. Any marginal advantage is super useful in that context and the well funded teams push to optimize everything. I think it’s more likely than not there is cheating. Motors seem unlikely but with this kind of money and international attention marginal advantages like microdosing for example will be exploited. People cheat in everything and often get rewarded for it. It’s an infuriating fact of life.