Video-game influencers are returning to Amazon.com Inc.’s Twitch en masse after seven- or eight-figure deals to livestream exclusively on other platforms ended. More than a half-dozen high-profile livestreamers started broadcasting solely on Twitch last year after contracts with Alphabet Inc.’s YouTube and the upstart Kick streaming platform expired. The gaming celebrities returned to a combined 25 million Twitch followers, according to an analysis by Bloomberg News. Several more are livestreami...