I have never seen a DEX focused build use two-handed polearms...ever. Fighters that don't use shields often use polearms because they don't usually have another use for their BA, and Polearm Mastery gives you a bonus action butt-of-polearm attack that deals 1d4+STR damage. (You can also apply use GWM with two-handed polearms).
Rules-wise: Polearms are defined as the glaive, halberd, quarterstaff, or spear. None of those weapons have the Finesse trait, and thus none can natively be used with Dex. The only exception is that monks can use quarterstaves and spears with Dex because those are Monk Weapons.
By "often" I can only assume you mean extremely rarely if ever at all. I've never seen a polearm using fighter...ever. It might have some merits I guess but it's not even remotely close to a top pick. I went out of my way to look this up and it's not even in the top 15 most used
by fighters, ok but wait...since the weapon is so good surely there's bound to be one polearm among the 17 best legendary weapons a barbarian might want, right? Surely at least one somewhere in there, even if at number 12 or something, right?
Nope, not a single one.
