How much do you "gain" by launching from up hight (e.g. the Andes / Himalayas, as opposed to sea-level like in Florida)?
The problem is that now you have to transport everything near and up the mountain, which costs a ton in logistics (getting a road up a mountain that can handle a entire assembled rocket isn't cheap, nor easy). Most launches have delta-v to spare because satellites are usually nowhere near the rocket's maximum payload weight.
tl;dr it can save 6% in a ideal situation, but the extra cost and logistical problems aren't worth it.
Unless you launch by means of a mass driver/rail gun [1] built into the side of Mt Everest with a piggy backed elevator for tourists. Tourists would arrive to a pressurized viewing area at the summit and have the option of donning an oxygen mask, going outside and posing for photos next to a cutout of Sir Hillary. I have never understood why serious rock climbing friends get so upset whenever I have mentioned this idea over the years.
At the proposed 5000mph, air resistance at sea level would be tremendous. Greater atmospheric losses would entail either bringing more fuel to complete the orbit, or larger launch loop / greater acceleration force.