Why are there full-fledged entertainment systems on aircraft when today everyone has a smart phone?
Here are some points to consider:


The screen size is a huge part of the experience. People with smartphones mostly still view content on a large regular screen when they are home or at a hotel etc. Hence, given a choice a passenger is always going to prefer the larger screen you can pack into the seat back than the relatively smaller smartphone display. 
If you can be sure every seat has an IFE screen you can use it for other purposes like mandatory safety announcements &amp; videos. Unless  you can be sure there's no seat without a smartphone owner depending on them for such messages is hard
If you have an existing aircraft the cost &amp; effort to modify it &amp; recertify the changed config. isn't worth it
Smartphones have only become ubiquitous in the last 5 years. 
Average fleet age is much more than that. A typical aircraft you fly on might have been built 10, 15 or even 20 years before today. 
Manufacturing date is not as important as the design date: If you are flying even in a 5 year old aircraft but the design was made 20 years ago there's no chance the designer could even forsee the smartphone revolution so he put in the IFE. 
New designs you can expect modifications e.g. Many recent aircraft have a local USB port you mention &amp; allow the passenger to plug in his device &amp; view his own movies or listen to his own music on the IFE screen. http://www.turkishairlines.com/en-int/travel-information/frequently-asked-questions/on-board/in-flight-entertainment



  Connections enabling passengers to listen to, or watch on their
  individual screens, content on their personal iPods or USB devices are
  available on B777 and some of our A330 aircraft.



The power and waste heat tradeoff of smartphones vs dedicated IFE isn't clear: If you encourage IFE via smartphones you would need to provide charging capacity for simultaneous plug in of hundreds of phones. The electrical resources for that aren't  insignificant. 
The devil you know is better than the one you don't: Dedicated IFE is tested &amp; certified with well known characteristics. Having hundreds of unknown chargers plugged in &amp; batteries charging adds other variables to the game. You'd also have to support multiple OSs, screen sizes etc. on the software side.