And yet there are quite a few sublcasses and features which deal with protecting or mitigating damage from others.
Let's take a look. Well you can choose a Fighting Style that uses your reaction each turn, but it competes with a lot of other fighting styles. There's the Cavalier, who gets their ability to protect starting at level 3.
But then you have stuff like:
Bear. While you're raging, any creature within 5 feet of you that's hostile to you has disadvantage on attack rolls against targets other than you or another character with this feature. An enemy is immune to this effect if it can't see or hear you or if it can't be frightened.
Hey that's a great defender feature! Too bad you get it at
14th level!
The issue I have isn't that there's no way to do it. It's that at each and every step, you have to opt into it, and most players don't, preferring to take options that boost their combat effectiveness. After all, the best status condition is dead, or so I keep hearing.
If you want a game where melee protects weak squishy casters, you can't say "well, as an option you can worry about the pointy hat, but it'll cost you something that'll make you more effective at murdering things", since not many players want to do that.
And if you make it so where they have to do that, and they don't want to, they might go and find another game to play. I don't like it, but that's just how a lot of people play the game.
I realize as I type this that I'm mostly (and a bit foolishly) arguing from a position of "this could be problematic for the game as it exists". It occurs to me though that as an optional approach to the game, something that individual tables can employ, as long as your players are on board, there's no real issue. Those of you who want this for your games- have at it, and I hope you have fun with it.
I'll stop tilting at windmills now.