Oil leaks can be caused by many things. First of oil leaks can be from anything that has oil as lubrication. Power steering, transmission or engine.
Gaskets, seals, O-Rings and hoses get hard and brittle over time. The engine compartment is a harsh environment due to heat.
It is important to change engine oil on a regular basis. Frequent oil changes keep the additives in the oil fresh and help the engine resist wear. A worn engine starts to create pressure in the engine crankcase and excess pressure will force oil out through weak gaskets and seals. For emission purposes these crankcase gases are recycled and burned in the engine using a PCV valve if this valve gets gummed up and sticks closed the engine will also leak oil.
Like anything things have a designed life span and seals and gaskets will only last so long.