Resuming all above, i can say that use cases of PC and phone are quite different. PC most times used in single or couple of apps (of course browser with bunch of tabs require many cpu cores, can lag even on top i-3), phones used to multitask. At least network connection, UI draw, system triggers, notifications. If you open task manager on PC there are many of processes too, but they use less than few % of cpu power even on old Core 2 duo.
4 cores is pretty cheap (MTK 65x2 was cost 1$ at start for OEM) It`s also RISK vs CISC when last lack on performance per core. Energy efficient != powerful, [as we can see here][1]. Multi-core is perfect for mobile,because no serious heavy single tread load, and multi-task aimed experience (but we can see that iPhones need less cores and RAM due to good software [like in this video or others][2] )  


  [1]: http://cpuboss.com/cpus/Intel-Pentium-N3700-vs-Intel-Pentium-3805U "as we can see"
  [2]: https://www.youtube.com/watch?v=3ajdZQc3q_8