Why is resting membrane potential or a membrane depolarisation localised to the membrane?

Even though it is called membrane potential, it's actually the potential difference between inner and outer cell, so in other words potential difference across the membrane. Since there is a potential difference of around -70mV in a standard animal cell (interior being the negative), it's normal that a bit more of the negative ions will arrange near the membrane, and vice-versa in extracellular space.

EDIT: I think I understand the thing you are asking. Well first of all, inside the cell there are impermeable anions (these arise from a variety of sources including proteins) that cannot go through the membrane. That helps keeping the negative potential difference a lot. Secondly, the attracted ions may diffuse to one anothers side as you mentioned, but that is compansated with Sodium/Potassium Pump which uses ATP to transfer 3Na+ ions out, and 2 K+ ions into the cells, thus helping the inside of the cell to keep its negative state.

The resting membrane potential is restricted to the immediate vicinity of the plasma membrane is because it is not a true potential in the strictest sense between the cytosol and the ECF, but is rather as a result of potassium ions continuously leaking out of the cell at the plasma membrane, leaving behind a local negative charge on the cytosol side of the membrane which does not significantly dissipate into the cytosol since the cytosol is close to electrically neutral (the negatively charged proteins cancel the positive potassium ions)