Test sites?
The 'problem' may be that there are no test sites at those altitudes.
Development of the perf charts for a helo isn't a 2 minute job and requires special facilities in order to approach the limit cases safely.
In the western hemisphere most of the testing seems to be done at Leadville at just under 10000 ft and that's the highest airfield in North America, if there are other places that could be used I am sure they would be, but I have heard of some locations in the Andes being used.
DM