Now listen, Debrid isn’t actually providing any meaningful bandwidth. It’s a third party (fourth party?) service.
What they are doing is simple: For their paying users (no clue what it costs without making an account, $3 a month?) they offer fast direct downloads. But they aren’t even storing the data themselves (besides caching)! They use premium accounts for other file hosters to get around the download throttling. So instead of you being limited to 1 MB/s or less for most downloads Debrid uses their account to download at full speed, then give you the file.
So they are pretty much abusing other hosters by allowing their own users to share a premium account for various file hosting platforms. Which will work so long until these hosters start aggressively blocking accounts that use too much bandwidth.
In addition to that you are paying Debrid money, $4 or something a month? If every YouTube user even paid $1 a month there would be zero need for ads. You are right, bandwidth is relatively cheap, but getting people to pay is difficult. Your suggestion would basically be that YouTube now forces everyone to pay $2 a month or they can’t access the service (or only 480p videos or whatever), which would work! But is far less suitable than charging more for no ads and have only one out of hundred(?) users pay while the rest happily watches ads.
If every user threw in some coins per month we could have services with zero ads. But even a cheap subscription like $1 or $2 is often too much to convert users. The service has to be free, so that out of a million users maybe a few hundred actually pay.