There are many reasons why this isn't always possible:So what?
Disengage from shielded character, Shield spell is not a WoW "taunt" so you must bash on it while it lasts.
if 22 AC tank is now 27 AC, I'm sure there are more squishier PCs to whack on for that round.
1) The creature doesn't have the speed to get to the backline. It either hits the tank in front or doesn't hit at all.
2) Narrow terrain, chokepoints, bottlenecks.
3) Grappling (this is what my party's fighter that uses shield does, grapple the monster, now it either spends effort breaking it, or just has to engage with the shielded tank)
Yes when possible don't engage the frontline, but in many fights you are pushed to do so.