No, Youtube does not gain copyright for several reasons:
- Youtube creates a derivate that is virtually the same as the original safe for the encoding. This means there is no transformation done, precluding fair use, and demanding a license, which you grant them in the ToS.
- Youtube does create the work according to a contract with the uploader, where YT gets usage rights only: it's in the Terms of Service! Therefore, while it might not be a Work for Hire, YT precludes itself from obtaining anything but the (rather wide) usage license:
License to YouTube
By providing Content to the Service, you grant to YouTube a worldwide, non-exclusive, royalty-free, sublicensable and transferable license to use that Content (including to reproduce, distribute, prepare derivative works, display and perform it) in connection with the Service and YouTube’s (and its successors' and Affiliates') business, including for the purpose of promoting and redistributing part or all of the Service.
License to Other Users
You also grant each other user of the Service a worldwide, non-exclusive, royalty-free license to access your Content through the Service, and to use that Content, including to reproduce, distribute, prepare derivative works, display, and perform it, only as enabled by a feature of the Service (such as video playback or embeds). For clarity, this license does not grant any rights or permissions for a user to make use of your Content independent of the Service.
Duration of License
The licenses granted by you continue for a commercially reasonable period of time after you remove or delete your Content from the Service. You understand and agree, however, that YouTube may retain, but not display, distribute, or perform, server copies of your videos that have been removed or deleted.
As a result, the uploader has all right to re-download it, edit it, and re-upload it, in fact, youtube demands that at times for fixing stuff into the guidelines. Youtube has some extra rights from the contract however.