Fluff: Why do ships always fight on the same level.

So, I was thinking, in all the fluff ships fly past each other launching broadsides at each other etc but there is no gravity in space. Why don't they fly under each other on their sides launching broadsides and the enemies underside and why don't they warp jump and come out pointing straight up towards the enemy to create a letter T, this would allow the lance batteries to fire at the underside and wreak havoc.

It's just a thought, try and prove me wrong either with a reason why they don't or a time when this happens in the fluff.

Thanks for reading,

Jacob.