Apple needs RF engineers because the RF inside phones becomes more complex over time with number of LTE bands and carriers and MIMO and Wifi/BT/NFC/GPS etc, and making all this work on their form factor. That is the same for all OEM who do not get "turnkey" reference designs.
Apple, like other, want to pick their own mix of Front End components, due to performance and pricing and feature set. The amount of testing also increases, and the production line calibration and testing needs upgrading with every new phone model with more RF antennas.
I know folks working on that at Aapl, they are not necessarily the best and brightest, quite the contrary in some cases. Aapl success has little to do with 'mobile' part of the phone, it is a commodity sourced from INF, QCOM, INTC and maybe even MTK or SS in future. Who knows.