Because the level 30+ will do increasingly more damage to enemies with ground slam. Let's go with your assumption that a 29 grabs the relic on Mars with you there? Guess what... That's immediately a wipe. Know why? The 29 can't one hit the hob goblins, but they can one hit him. As a 30+ you can one hit all of them. Next? Venus.. The 29 might get the goblins, but the minotaur won't even flinch and stomp the 29 dead. Are you stupid, or do you just not play much?