A few things quickly off the top of my head:
Defenders are built in a way that allows them to be stripped down, rebuilt, added to and modified some may say they are built properly (well once we get in amongst them and sort them out) even the new one, everything can be taken apart and rebuilt, new chassis new bulkheads. Try doing that with a Disco3 shell, once its rotten through its done with, or spend a LOT of time and money welding.
You have axles on Defenders, this makes running gear repairs so much easier, it has an advantage offroad no matter how good independent units are.
If you run into a tree in a defender you take the bumper off and hit it with a big hammer, or you get another one for £50.
Defenders can have more done to them, think of all the mods that you have ever seen to a Defender then imagine trying to make these fit a D3 and keep it looking the part. Take winches, you have to butcher bits on a D3 not a Def.
The majority of Defender drivers in here have them for the above reasons and many many more, and that is why they would rather have a defender, it has been the leader when it comes to offroading all its life - and before in its series generation. They are brute force and ignorance and they get the job done.
You are asking this question to people who want to do things themselves, they want to be able to undo lots of bolts and it comes off, not all in one welded body shells. These things may seem trivial now but the type of vehicle a Defender is, people will buy one and probably run it for years, so they are thinking to the future when work will need done.