I blame Oblivion for the trend in "It's okay to ignore one of the most basic weapon types in historical settings, Fictional or otherwise."
The 'real' reason is probably how different Spears and polearms are in terms of the kind of assets you need to create, almost none of the animations of even two-handed weapons translate over to the proper handling of a spear or polearm, mostly because your "Main" hand is at the back of the haft providing power with the trademark distance while the "Off" hand is used fore grip for stability. To be honest, the handling of a Rifle-grip weapon has more in common with a Polearm than a battleaxe, despite the similarities both weapons exhibit.