Resting potential occurs because

Resting potential occurs because of the electrical potential difference across the resting membrane.

The ionic gradient across the resting membrane is maintained by the active transport of ions by the sodium-potassium pump (Na⁺/ K⁺ ATPase) which transports 3 Na⁺ outwards in exchange for 2 K⁺
into the cell. As a result, the outer surface of the axonal membrane possesses a positive
charge, while its inner surface becomes negatively charged and therefore is polarised. The electrical potential difference across the resting (unexcited) plasma membrane is called as the resting potential. Resting potential ranges between –40 to –90 mV. A typical value of resting membrane potential is –70 mV.