Differentiaion is definitely one reason OEMs design their own ASICs, but there are many more: 1.) Design control 2.) Long term supply stability 3.) Price. 4.) Power 5.) Cost 6.) Performance 7.) Density 8.) IP Protection 9.) Time to market
There has been a lot of hype in the last N years proclaiming the "death of the asic", and yet there are still plenty of monster ASICs being built by tier-one vendors.(especially in the comms field) Sure, ASICs are now more expensive to build ($5-10M) but they still make sense if they can be used to gain or keep revenue shares in end equipment markets that are in the $B's.
Maybe ZTE wants to better protect their design from a copycat. If they used a standard SRAM-based FPGA and a standard DSP it would be easy for someone to copy or reverse engineer their design. With their own ASIC they can better protect their design from possible theft.
The Other Tesla David Blaza5 comments I find myself going to Kickstarter and Indiegogo on a regular basis these days because they have become real innovation marketplaces. As far as I'm concerned, this is where a lot of cool ...