Vague steering that requires constant corrections on the highway is genuinely tiring and worth sorting properly rather than accepting as normal. Tie rod ends and steering rack wear are common causes of exactly what you're describing and neither shows up on an alignment reading alone. A proper inspection involves checking each component for play and movement under load not just measuring wheel angles. Knowing exactly which part is worn before agreeing to any replacement saves money compared to guessing and replacing things that aren't the actual cause.