Because you actually have to pay for google drive to host your videos on it, and the storage space is pretty limited. You don't need to pay anyone to store videos on youtube.
Google Drive is a paid service with different subscription tiers available depending on how much storage you are using. I know that video uploads count towards that storage limit, and you can easily test it yourself. Try uploading a video, and see it counting towards your Google Drive storage limit.
Then logically, why should it be available ad free either?