how can growing the same crop repeatedly on the same field deplete the soil?

It requires nutrients to grow...  nitrogen, phosphorus, water, oxygen, iron, etc and these get used up especially if you are growing the same thing since it needs the same nutrients.

So, if you keep on taking money out of your bank account wihout putting more in, what will happen (money depletion).

Here's my take on it anyway.

The reason growing the same crop on the same field will deplete the soil is that crops require certain nurtients.  When plats grow, they take in nutrients (and minerals) from the soil.  This of course means that each time a plant is grown in the same soil, more nutrients are taken out of it.  This is especially a problem with the same crop because the exact same amount ofeach kind of mineral is needed for growth, and therefore depletd more.  However, the main nutrient provided by the soil for growth is Nitrogen, and all plants need that.  Therefore, Nitrogen is the main, but certainly not only, nutrient being depleted.
The simplest way to fix this is to grow a legume, preferably soybeans, at least where I live.  Soybeans actually add Nitrogen to the soil, because (i am not certain of this) the Nitrogen-fixing bacteria on their roots produce more than the plant needs, and it therefore gets deposited in the soil.