That's because Udemy seems to attract more MARKETERS than real instructors (i.e., those who have a solid track record in their professional field). But, like you said, that doesn't mean there aren't diamonds in the rut. You just have to look for them.
I mean, marketers are everywhere on the internet and selling digital property for profit has been the buzz word among them -- thanks to Udemy's own marketing team. Add that to Udemy's algorithm for getting your course featured for organic traffic, there's going to be a lot of people keen on quantity over quality. Especially when they know they are going to give away free coupons to attract 'social proof' buying. For a company like Udemy to be successful, it needs a lot of instructors for students to choose a course from. So they have a dedicate team of recruiters asking youtubers and just about anyone trying to make a buck, to upload their course on their website. The sad part about that is the real gems can get easily buried among the wannabes. Especially if some reputable teacher decides not to play the 'social proof'/giving-away-free-coupon game.