So we've mentioned a couple of reasons so far which I'll list:
- we want launch sites on the coast rather than inland so anything the detatches from the ship falls harmlessly into the ocean
- it's more costly to build a launch site on a mountain where you have to transport everything up and down the slopes.
I think it's also important to point out that even the tallest mountains only reach less than 10,000 meters above sea level, and the lowest satellite in low earth orbit is approximately 160 km above the earth's surface. So you'd be doing all that work on a mountain top with more unpredictable weather and harsher navigation just to trim off less than 10 % of the fuel you need to get to LEO.