Short answer: Because a peanut is a seed (a legume), not a root.

• In Jain dharma we avoid kand-mool (root and tuber vegetables like potato, onion, garlic, carrot, beet, radish) since uprooting and cutting them involves more himsa and countless micro-lives (anant-kaya).
• A peanut (groundnut) grows inside a pod under the soil, but the edible part is the seed, not the root. Eating a detached, dry seed is considered less himsa than eating roots. Therefore, peanuts are generally vihit (permitted) in Jain food.

Notes on practice:

• Both Shvetambar and Digambar traditions commonly allow peanuts.
• Some families still take a personal vow to avoid them, or avoid them during special vrata (e.g., during certain days of Paryushan/Das Lakshan). Respect such vows.
• Prefer dry/roasted peanuts, avoid sprouted or moist ones, and check for insects. This keeps himsa lower in daily conduct.

So, even though they mature underground, peanuts are permitted because they are seeds, not root-vegetables.