Well the people that built those cathedrals generally kept the techniques of masonry a secret. Sure, people were free to use the fruits of their labors (for various shades of free) but that knowledge wasn't generally made available to all when the cathedral was finished. If you wanted another cathedral you had to go to the same (or similar) people who built the first one.

Masons of the time closely guarded their secrets, like source code from a vendor like Microsoft, Oracle, etc...