I mean the trees are an abstraction of a physical terrain piece with space for the vehicle to maneuver through.

IRL the scenery would have like 3 slender trees and huge gaps to move models through, but in TTS everything has to be individually modeled and resized/reoriented for implementation on a map, so its easier to simply turn off collision and say vehicles can pass through so that assets can be reused between maps.

Making maps is pretty time consuming as is without also having to create bespoke scenery.

What part of the rules says you can maneuver vehicles through woods and that the trees are an abstraction? I think you're thinking of editions previous to 8th and I've never played it like Rhinos can ghost through a forest in 8th, but then because trees provided very little benefit in 8th I went out of my way to not play on tables with trees.

The trees don't have collision. In TTS, everyone just plays it that you can maneuver vehicles through woods, since its really just an abstraction and you can just pretend the model has space for the vehicle in a ring of foliage.

The forests are incredibly difficult to move through. You cannot place a Dunewalker on top of a tree AFAIK.

I removed the central light cover obstacles to allow vehicles to more easily pass between the ruins, but I worry that might leave some side objectives suicidaly exposed.

What base sizes struggle? I tested it with a Dunewalker and a Rhino and they had no issue moving around.

Too much terrain, it would be almost impossible to use vehicles on this map.