Because the UX focus is different, the assumed technical infrastructure context is different, and, well, basically everything except the backend service consumed is different. Youtube Go is more distinct in role from the core Youtube app than the other break out apps (which mostly are distinguished by content focus on UX tweaks around the focal content) like Youtube Kids, Youtube Music, and Youtube Gaming.
Because the UX focus is different, the assumed technical infrastructure context is different, and, well, basically everything except the backend service consumed is different. Youtube Go is more distinct in role from the core Youtube app than the other break out apps (which mostly are distinguished by content focus on UX tweaks around the focal content) like Youtube Kids, Youtube Music, and Youtube Gaming.