Here's why:

* Tendrils: Pea plants have tendrils, which are modified leaves that wrap around supporting structures like fences, poles, or other plants. This allows them to climb vertically.

* Growth Habit: While some pea plants might initially grow horizontally, their main growth pattern is upward, using tendrils to climb.

Creepers are plants that grow horizontally along the ground, often rooting at nodes. While some pea plants might have a creeping growth pattern if they don't have anything to climb on, they are fundamentally climbers.