Why are there no mountains higher than ~10 km?
The reason mountains are higher on Mars than on Earth is the lower gravity on Mars, which is only one third of Earth gravity. Mountains are very heavy things, and on Earth the forces that build them have to fight against gravity, which is constantly trying to drag them down. For the same reason, small asteroids are usually not spherical, because they don't have enough gravity to even out the bumps, but when they get to about the size of Ceres, which is 500 miles in diameter, they do. Ceres is spherical. Mountains like Mauna Kea which have a large part under water don't count, because although mountains are heavy things they still have buoyancy and the sea is taking part of their weight. Venus has slightly less gravity than Earth, but because of the enormous heat is also more plastic, so this would mitigate against the building of very high mountains.