Earth has never had much higher mountains than the highest now existing. The reason is that when the mountains get high enough they start to crumble because the rock at the base is not strong enough to take the pressure, and they start to flatten.
In addition, the crust "floats" on the mantle, and a massive mountain pushes the crust down, causing it to sink, limiting its maximum height.
The theoretical max height of a mountain on Earth is around 10-14 kilometers (6-9 miles) above the surrounding plain, limited by rock strength, gravity, and isostasy (crust sinking under weight).
But since erosion works away at the mountains as they are formed, Most mountains never reach the theoretical maximum. The tallest on Earth now is Mount Mauna Kea (over 10 km base-to-peak), close to the theoretical max
.Footnotes