山 is a picture of three mountain peaks rising from the ground — the tall middle peak flanked by two shorter ones. It has barely changed in 3,000 years. When you write it, you're drawing the same skyline the ancient Chinese saw.
Mountains are sacred in China — home of immortals, temples, and poets. The Five Great Mountains (五岳) have been pilgrimage sites for millennia. 山水 ('mountains and water') is the name of an entire genre of landscape painting and the Chinese ideal of natural beauty.
Three peaks — a tall one in the middle, two shorter ones beside it. A MOUNTAIN range.
Stack three 山 on top of each other and you get a real character: 嵾. And 出 (to exit) looks like two mountains — it once depicted a foot leaving a cave in the hills.