The main reason is the time it takes for the active chemicals in the cell to diffuse into the electrodes and reaction products of the chemical reactions to disperse.
The active chemicals are consumed when the cell is being discharged and waste products produced.
This is closely related to the ability of the cell to provide high current discharge without excessive voltage drop.
When the cell is being discharged the chemical concentration within the electrode becomes lower as they are consumed and reaction products build up thAT slow further reaction. This causes a drop in the terminal voltage. Active ions from electrolyte at a distance from the electrode will then diffuse from more concentrated regions to the active region but that only occurs slowly within time scales of minutes. As the concentration returns to normal the open-circuit voltage recovers.
As Winny's answer describes this diffusion can be modeled as RC time constants when creating an electrical model of the cell for simulations. It may take multiple capacitors and resistors to create an accurate model.
The question doesn't state what type of cell is being referred to but all types suffer from the problem to a greater or lesser extent the details of the chemical reactions will vary. The rate of recovery will also vary with temperature and the mechanical construction of the cell.
In Leclanche type cells such as zinc-manganese dioxide (alkaline or zinc-carbon cells) the effect is known as polarization. Hydrogen gas is created by the chemical reaction that impedes further reaction. It takes time for the manganese dioxide to oxidize the hydrogen to water.
Lead-acid car starting batteries are another example; they are designed with thin electrodes to allow them to provide the high currents for starting without the voltage dropping too much because the sulphuric acid only has a short distance to diffuse. That does however result in a less robust battery with a shorter life; deep-cycle batteries have longer life but less capability to provide high-starting currents.
For lead-acid batteries the effects of diffision are expressed in the form of Peukert's Law (Wikipedia) that relates how the effective capacity of the cell varies with discharge current.