Exactly!
Also addressing the OP's questions:
1. Because we can and you can't.
2. The same reason you built the car you have. Personal satisfaction
3. We try to use the most original parts we can, if they can't be found we have to use what is available. Ever try to find original, date coded, with the correct part number trunk weather strip?
4. The never ending search for truth and recreating what was and what will never be again.
5. It's fun!

The same reason an original untouched 1500 mile car sells for a butt load of money is no body else has one! It is only original once. Since there aren't 1000's of these lying around, the next best thing is to have a restored car as close to being in the original condition as humanly possible.
I could go on and on. I guess a return question would be why spend all the time and money on a car like yours, that isn't original. You will never get your money back and if you try to sell it someone has to love your personal taste. If it is original thousands of people want it.
I am always amazed that I never see posts from restorers, paint dab , numbers guys bashing the Hot rodders. I love all mopars in all condition. I am happy you are doing what you want to your car. Yet we always see hot rodders, racers, etc. always bashing the numbers, resto, car show people. If you are happy doing your thing, I am happy for you. Why is it so hard to be happy that we are doing our thing? Nuff said