They don't have the bandwidth to do it. They can't keep up in hardware design, CPU manufacturing (TSMC throughput), software, anything. They're running on all cylinders just trying to release a new phone, new OSes every year and in addition to that chasing the dream of creating some kind of new category (TV, watch, cars, VR headset) that they can't afford to assign engineers to something mundane that they can silently outsource.

Unlike companies like Google, Meta and Microsoft they are institutionally afraid of over-hiring and losing their culture. This is a conservative tradeoff where they may lose the ability to grow seemingly "small" markets (small=anything smaller than the iPhone) in favor of not losing their culture (becoming a Google or Meta which just squanders every opportunity)