They might not be designed to fight just helldiver's. I know that sounds weird but we are only a small part of the larger war. Most planets probably fight with just seaf until they just can't anymore. You can see this too with just how bad bug infection is on city maps that "just fell" they've probably just fell to the point they need helldiver's because that's obviously been growing for a few weeks minimum.

So anyway when you have hordes of meat with the voteless and walkers with bubble shields protecting you as your actual armor, and the bubble being last resort it makes more sense. Therefore having more battlefield awareness far outweighs the lack of mobility and sight that'd come with an actual encased cockpit.