Here's why:
* Adhesive is Key: Fake nails (acrylic, gel, or press-ons) require a strong adhesive to bond them to the natural nail. This adhesive is typically a specialized glue or acrylic/gel liquid-and-powder system.
* Polish is a Coating: Nail polish is designed to adhere to the nail surface and provide color and shine. It doesn't have the bonding properties necessary to hold a fake nail in place.
* Potential Hindrance: Applying nail polish *under* fake nails can actually *prevent* the adhesive from working properly. The polish creates a barrier between the adhesive and the natural nail, weakening the bond. In some cases, it may be used after a set is cured to protect the nail or add extra detail/bling.
In summary, use the appropriate adhesive recommended for the type of fake nails you're using to ensure they stay on securely.