I’ve always wondered why this is? In our country a previous prime minister remains highly relevant and politically active until they retire, even if that is long after they were in the position. The leader of the current largest opposition party was prime minister for eight years before losing the previous election, and is set to be the opposition’s front runner for PM in the next election in not too long.
Like, why didn’t e.g. Obama run for a position on the senate after finishing his second term as president? He’s definitely still young enough, even in countries where you don’t need to be a fossil to have political power.