"Why do you say that?"
If you have to ask that, you have no clue and it's almost not worth explaining to to you.
Sorry, not being rude at all, just don't have a spare week to go into everything with you.
I've owned twelve Karmann Ghias, have never been without one as my daily driver in over 35 years. Pretty much know everything that there is to know about them, even act as the judge at the Litchfield and Terryville VW Shows for the Karmann Ghia class. They're of an extremely complicated body construction, over-lapping seams and body panels leaded together, with multiple inner structures throughout. Each of these sub-structures is prone to collecting water and rusting from the inside out, with no factory rust prohibitor or undercoating. Cabriolets are simply metal bowls, coupes are little better. When the seals go, they just sit and collect water, so much so that owners very ofter drill 5/8s holes in the floor pan at the lowest spots to help drain it off.
Multiply what I just "mentioned" by one hundred and you'll only scratch the surface of how difficult these cars are to restore, especially at this late stage in their existence.
Example: Guaranteed that every body-to-pan bolt is rusted in place and will snap if you try to unbolt the body from the platform, the hidden forward bolts will be corroded beyond approach and the rear bolts that go through the small "pads" on the cast rear shock towers will be an even bigger headache. Now you need to either drill and tap every hole or most likely replace the pans and lower body sections that carry the threaded holes and mounting holes. Of course, this will mean inner/middle/and outer rockers, heater channels, pans and perimeters, front and rear Napoleon Hats, battery tray area, rear inner body section, fire wall base, lower lock plates and hinge plates, and possibly new front foot wells.
THAT'S, just what will need replacing if the areas holding the body mounting bolts are bad. All of this is well below the belt line, haven't even considered the actual body, mechanicals, soft parts, rubbers, and on and on.
If you can get the body stripped, disassembled, and off of the pan in a day, head for New England and you'll make a fortune.
Expensive nightmares to do right; flimsy, dangerous, crap cans when done wrong.
Just stay away, more for me !