This same ease in transport and marketability applies to investment grade diamonds. Had a client dealing in these back in the 1980’s.
That’s true. One can easily move diamonds across borders.
There is a problem with diamonds though: many people will recognize diamonds, including border officials, immigration agents, and TSA. Sure, one can hide diamonds in many ways (some covert and intelligent, otherwise straight-up disgusting …), but if seen, almost everyone will recognize diamonds.
Not the same with watches.
It is very easy (VERY) to take a very expensive watch through the most stringent immigration checks with very little chance of being discovered.
For example, one can fly anywhere with a Patek Philippe 5711/1A-018, zero questions asked, and that’s a watch that has a market price of over USD 1 million!