Why does he have to bind?
It's the rule.
I’ll give you a clue
He doesn’t
Joining a ruck
- An arriving player must be on their feet and join from behind their offside line.
- A player may join alongside but not in front of the hindmost player.
- A player must bind onto a team-mate or an opposition player. The bind must precede or be simultaneous with contact with any other part of the body.
- Players must join the ruck or retire behind their offside line immediately.
- Players who have previously been part of the ruck may rejoin the ruck, provided they do so from an onside position.
- Sanction: Penalty.
https://www.world.rugby/the-game/laws/law/15