Why isn't smurfing a bannable offense?

As a bronze player, I work tirelessly to escape hell. Yet I am oppressed. Promos are plagued by DCs or smurfs who feel they are there to prove some point. Riot, how are we meant to improve as players, let alone climb, if we are kept down by players who are full divisions better than us? Shitty or toxic teammates are part of the game sure, but smurfs? why? why is this ok? I am essentially being told I must be better than gold or plat players in order to escape bronze. clearly the logic is missing.