Since the beginning, the fighter's role was front-line infantry, while the wizard was artillery. In the olden days there was a sort of unwritten rule that many, even most, monsters would gravitate towards attacking the PC in the front of the party, or the 'one who looked toughest' (the fighter, fighter-sub-class, or, later, barbarian). In CRPGs/MMOs it was hard-coded as 'aggro.' In 4e it was formalized by mechanics that supported the defender role. In 5e, there's one fighter option a little like that, but it's mostly back to counting on the DM to control the monsters in the way that makes the best fight, not the way that fights best (and can result in excessive character death). Players can help by sticking to a defensive, squishies in the middle, marching order, and pushing their fighters into any convenient choke-point, like a narrow corridor (or two abreast in a standard 10x10 corridor), doorway, or the like.
Yes I understand what a tank role is. I'm saying, if your group just popped up out of a chimney that the scout/rogue went up first, and haven't had time to organize yet and get ambushed by a bugbear, how exactly is it the fighter's fault if the bugbear attacks someone other level 1 character that has less max hp than the suggested bugbear attack hits for?
You're telling me you expect the DM to have monsters only attack the fighter? I find that kind of lame. I spread the attacks around if the players have their characters in attackable positions and haven't done anything to avoid being attacked.