It depends on your start date.

CK2 is a game and needs formal mechanics that allow you to expand your influence and rule. Real life tends to move by largest army diplomacy where rulership is determined. Claims on territory don't disappear over generations, at least not when you're friends with the pope (or have your own pope, or have an army outside Rome). Up until the late medieval period countries with set borders weren't really a thing and rulers were loyal to other rulers until they weren't.

Charlemagne, for example, constantly travelled from region to region to improve relations, sway new people under his rule and fight over whatever lands he'd set his eyes on. Then he would either settle the matter of vassalisation with the existing ruler, or install a new vassal (though this would be more cumbersome and likely result in less stability) When he died his lands were split between his heirs (all hail gavelkind), but the vassal lords didn't stay loyal to these new kings because they'd inherited these realms, but because they were swayed one way or another.

Waging war to impose your rule on nearby territory, to then hand it over to a vassal to increase loyalty may be done, though.