Why did portugal build trade forts on the west african coast, rather than go into the interior to trade?

Portugal created trade forts along the coast rather than inland because their goal was to control trade routes. They were not looking to build and control a land empire. They were after the route needed to gain access to the East Indies, India, and China. All they needed to control was the coastal area. 

Trade forts enabled Portugal to ensure that the supply lines remained intact, as well as to protect ships taking on supplies. In addition, forcing local suppliers to 'bring goods to market' protected Portuguese traders from potential attacks. In a similar vein, it reduced the risk to the Portuguese traders of damage to the 'merchandise' during transit to port, as the suppliers would have borne that burden up to the point of delivery at the trade fort.