Why aren't spacecraft launched from higher altitudes?

This is a question that often gets asked, and the other day I noticed a very good response to it on Reddit. The short answer is that it's worth more to have a launching platform closer to the equator in order to benefit from the Earth's angular momentum, as it's highest at the equator. This is why the US uses a location in the south (Florida), the EU uses Guyana, Korea and Japan use the south as well...but also note that it's not worth it to have a launching platform in the middle of the ocean as transport costs then skyrocket, which is why the US uses Florida and not Hawaii, Korea uses Goheung and not Jeju-do, Japan uses Kagoshima and not Okinawa.

Ideally it would be best to have a location at the equator with a location easily accessible and of high altitude. In fact, there is a place of that nature: Mount Kilimanjaro.

It's only 3 degrees from the equator, is 5,882 metres in height, and has a flat top. You can also a comparison of it here with other mountains and how tame the slope is to your average mountain. At the same time it's still so high that travellers get altitude sickness, with an atmospheric pressure less than half that at sea level. The only problem is that it's not located in the US or in a major industrialized nation - it's in the middle of nowhere in Tanzania. If the US was located where Tanzania is you can bet they would have already made good use of the mountain.

At the same time though, it's probably not worth it to pursue exotic locations to launch rockets as it is probably more cost-effective to develop combination balloon-rocket launching systems, where a balloon or other craft carries a rocket up to some 20 km or so above the ground, the rocket separates, and then fires to take itself to space. Even all-volunteer companies like JP Aerospace are capable of doing this (but not putting the rocket into orbit) as you can see in this very grainy video.