Everything Is Big On Mars
Mars has no active tectonic-plate-like activity. While this means that mountains will not form through new activity, it also means that there’s no movement of the surface that would over time equalize it as the vertical features are “shaken” to be more flat. With tectonic activity, valley systems would not be so deep and remain stable.
Another effect is what keeps planets mostly round: gravity. Gravity imposes a limit to the height of a structure over time. Mars has a much weaker gravity, and can therefore have taller mountains remain stable over time (Olympus Mons).
There’s also no significant erosion to smooth out features like Valles Marineris. Some of those large depressions can be the bottom of ancient oceans, and when you are looking from the bottom of the ocean the highest mountain looks that much higher.
So, no tectonic activity + low gravity + no erosion = vertical structures are more long-lasting and stable. Mountains, canyons, and other geological features on earth shrink over time, as water, wind, and biological erosion wears them down. Mars has very little atmosphere and liquid water and no known life, so erosion is much less intense.
Also, gravity causes mountains to collapse up on themselves and spread out. Less gravity means mountains can get taller before they collapse. It also means no new features being formed to erase the old ones.