they do such a thing cause all you fucks out there that buy a car want it to be quiet and never smell anything, so they spend millions on NVH testing, which drives the car prices up and adds components and complexity to an otherwise std car.
so they encapsulated the whole assembly, the HVAC system is the easiest path for noise and smells to enter the cabin.
instead of making hatches and shit they make one easy to install panel that is installed at the assembly plant and 100+k miles down the road when a part fails you cut out the access panel, if people werent retarded you can cut that whole panel off in one piece and just rtv it back into place afterwards, this isnt rocket surgery
hell go try and replace a heater core or an evaporator on almost anything nowadays...you have to cut the HVAC box apart as they plastic weld the assembly together so that the highly skilled HS dropouts can assemble the car faster and save the company 1$ per car.....that they had to spend on NVH testing to make sure people wouldnt complain that their brand new 20,000$+ machine sounds like a machine.
i need to start smoking again