There are a lot of factors that feed into vehicle design. It was more economical to begin with, requiring less material, and in a war economy that was always important. It was often more practical; in the case of large self-propelled guns, a full casemate would limit visibility, gun traversal and elevation, would require venting systems, and would not be thick enough to provide much protection, as chassis were often overloaded as-is thanks to their large main armament; if they fully invested in the same sort of design and thickness of armour of contemporaneous tanks, then they would essentially be replicating another design and there would be no saving of effort or material. The vehicle would be slower, have higher ground pressure, and so on.

When it comes to the threats mentioned, tanks were still vulnerable to artillery fire. The thinnest part of their armour was often the top, and a direct hit from an artillery piece in the indirect fire role could take out a tank. This was unlikely in practice because artillery is not a precision weapon. Artillery pieces are deployed in groups against area targets. An open-topped vehicle's best defence against artillery fire was mobility, displacing before artillery could respond to their presence.

Grenades are only a problem if the vehicles are within throwing range. Supporting infantry's presence generally dissuaded that, albeit in urban warfare isolated vehicles of any kind were very vulnerable. Being open-topped exacerbated this vulnerability, but tanks did not have any special immunity.

Aircraft were feared, but actually only a small percentage of vehicles were destroyed by aircraft. Pilots were infamous for claiming kills far in excess of what they actually inflicted. While their weaponry (cannons, bombs, rockets) was certainly capable of tank busting, hitting those targets (low altitude, high speed, low accuracy) was difficult and again, mobility was a better defence over a few millimetres of armour.