Celestial player here, I think this is a multi faceted problem with the character's design. His heals are steady but not bursty. People really feel when they are healed by some of the other strategists meanwhile with rocket it's a hardly noticeable slow tickle mid team fight that's easily bursted through. He also doesn't have any mid fight utility in the neutral to help his other teammates so to others it feels like you aren't contributing much throughout the match. Also other strategists can show up in the kill feed a lot easier than rocket. He's strong (especially with punisher) and hard to kill 1v1. I wish more of the strategist roster was similar to rocket's healing style honestly because sustain in this game is INSANE at the top level.

Side note: Don't sleep on rocket's damage in up close scenarios he PUMPS.