Why are some simple food over priced at restaurants ?

The basic reason is that even the cheapest item they sell has to contribute to fixed business costs.

Staff wages and employee benefits, training, licensing, cost of premises and equipment, cleaning maintenance and repairs of equipment and premises, staff uniforms and laundry, banking and accountancy fees, loan interest and repayments. The list goes on and on before even a hint of any profit is added.

Even items they could obtain and serve at absolutely no net cost to the business still have to carry a share of those costs.

Whether sales are made or not, those costs accrue every day. 

Restaurants may well choose to exempt some items from directly contributing to those costs as either promotional items or “loss leaders” which are both methods of trying to more than offset those losses by generating greater sales. It’s why strictly speaking, restaurants should even charge for tap/faucet water which costs them almost nothing per glass; they still have to buy, wash, and periodically replace the glasses/jugs it is served in, and it takes staff time to serve it and clear up afterwards. It also can be used by diners to substitute for buying beverages. But it is usually supplied free on demand because diners expect it and would take their custom elsewhere if forced to pay. Many still choose to pay instead for expensive bottled waters.

Because the restaurants are in business to make a profit. 

Like fries being $4.95 or higher when it's basically just one potato, potatoes that they get in bulk mind you, and some salt?  It's a very long story.   

Because its more of a fine dining restaurant and depends on the location