Not sure Cobra people change cars more often then fx Corvette people... Your statement is not a fact, but your personal opinion
That said, I "think" reasons are:
1) You spend years building your Cobra... and then you are off to the next Cobra project ( don't forget Cobras are not "off the shelf" cars )
2) Change in personal life
3) Those who sell fast... wrong car - bought a z06
From a personal perspective...: I got my Cobra 3 years ago after reading for 2 years - Like you I also made a list to build my car "exactly to my specifications and I'm really looking forward to it"... BUT dreams change
If I were to build my Cobra today, it would be different...
I would suggest you buy a used one - drive it for a year, and
then build your dreamcar
Good luck
Henrik