Capturing HQs

[Liebchen]

If you can capture an enemy HQ in reality, don't you in essence surround them and prevent them from getting away?

What you are describing is, indeed, "surrounding" and not "capturing" because the HQ unit in question is still a cohesive unit complete with arms (if not the training and experience to use them effectively).

"Capturing" would entail the captors' disbanding the unit. Since much of the combat losses in the game represent surrendered, fleeing, or lost combatants (and not necessarily dead ones), then elimination of the HQ unit is really the only good simulation of capturing it.