Speaking only from my own experience (read )
In our own humble way, we are trying to recreate the car that Chrysler gave us 44+ years ago.

For different guys that means different things.
For the absolute OE crowd that includes markings, paint daubs, properly finished bolts, parts etc....

For the resto-mod crowd it's keeping it stock appearing but adding
on upgrades/goodies as they see fit... Resto mod is really personalization. The car is part of them... not chrysler history..

Outside of that...why do they do it?
the most obvious answer is...because the can and want to....
it's a fun/challenging (and very expensive) hobby....
In so doing, it allows us (if only in our minds anyway), to relive yesterday and hopefully in the process we get a car we can actually drive....

You can't measure the value of the restoration (resto mod)
experience/effort based soley on resale value of the car, anymore
than you can place a value on golfing by the golfers scorecard....

There is simply more to it than that.
Last I checked...nobody makes us do this...
we do it because we love it....

Do you really need any other reason that that?