There's a big difference between YouTube fame and Twitch fame. Counterintuitively, there's actually little overlap.

Relaively few YouTube gaming celebrities stream due to quality control. Twitch celebrity streamers use YouTube primariy to host stream highlights as Twitch's VODs are not convenient.

Notably, YouTube Gaming, which was YouTube's Twitch competitor, failed miserably since there was no clear advantage to switching from Twitch (among UI/Content ID/monetization issues)

I know many YouTube gaming celebrities who have become successful enough for merchandizing, but not any Twitch celebrities. They're separate markets.