The air is less dense, which is why air-breathing engines and wings and propellers don’t work as well at higher altitudes, all other things being equal (nor do lungs, but that’s a different discussion).
Since an electric motor doesn’t care about the less dense air (with the caveats discussed above) one has the ability to work on the wing and prop without also having to deal with less available shaft power. What one needs is more thrust at speed, rather than more static thrust (mass is the same, drag is lower in the less dense air). The wing needs to go faster to develop the same amount of lift, and adding prop pitch is an easy way get that, given that the wing is going through that lower density air.
I did, when I was doing these crude tests, I did also do static thrust measurements via a digital fish scale hooked to the horizontal stabilizer of the airplane, and generally tried to add pitch to the point I got similar values to that at sea level. I would have to go back and find notes that are at least two decades old to see about static RPMs in these situations. I just don’t remember.
This is also why those seeking rocket altitude records (see Adrian’s threads and others) like to launch from high altitude fields. They don’t have to deal with the first, say, 8000 feet of dense atmosphere and of course a rocket motor, being a rocket and carrying its own oxidizer, also doesn’t really care about lower density air for its performance.
