Reverse engineering can be quite difficult. Because even when you have hundreds of workers knowing what procedures they have to follow, they mostly don't know what the exact shapes of parts and the material they are made from are important. You could get a number of workers who worked on a certain type of ship and ask them to do the same thing they always do. But without a superviser who understands how all the components work together and influence each other, the result would be less than spectacular. And in antiquity and the middle ages, getting craftsmen from a specialized trade from a foreign nation may not even have been that easy. To build a warship for example, you might need easily 20 or 30 workers who each are familiar with a different part of the construction, and getting one of each type to betray their country and work for the enemy might not have been easy.
Even NASA had to go to scrapyards to find pieces of their Saturn V rockets because in the space race documentation was quite shoddy and engineers had no knowledge of why their predecessors used certain materials and shapes as specified in those plans they had not been thrown away.

And I think for most of history, there were no standard sized components and exact sizes were decided for the specific project and the material available.
Only the architects and engineers would know which factors are important and where you can make compromises regarding material or size. And there probably were not many of those people around. After all, that's the source of their income. If there's ten people who can build a certain type of warship, they can demand very high pay, especially when this technology provides a vital advantage for their employers. If the technology becomes public knowledge, then you can hire the engineer who works for the least amount of pay and when everyone can have it, it might not even be that useful to employ anymore. It's a cartel, if you want. And agreeing to produce that technology exclusively to a single nation can push the payment even higher.