system
Given the following post, respond as if you're commenting on Reddit:
human
[Halo] Okay, who is the idiot who designed the UNSC vehicles?

Seriously, every land vehicle is just designed wrong. The Warthog has no doors, the Scorpion can get hit by everything, and there are literally 21st century ATVs.

It doesn't matter if you spend more time in the Space area than ground, you still have to make military vehicles make sense.
gpt
Most of the current run of UNSC vehicles were designed to fight the Covenant against whom armour is mostly useless. Warthogs have no doors because the extra maneuverability gained by the reduced weight is more beneficial than the marginal defense added by the doors. Same for the ATVs. 

Against plasma weapons you ideally want to avoid being hit in the first place than try to absorb the damage since the plasma doesn't bounce or ricochet off. Instead it sticks to what it hits and burns constantly until it cools down enough. The only materials the UNSC has that can effectively dissipate heat of that magnitude at any rate that would be useful are super expensive. The Mjolnir armour is made with it and even that can only handle so much, and each suit costs about as much as a Frigate. Shields would work too but has the same issue.