First off designing a whole core architecture from scratch is a huge and very expensive undertaking. Youd need to be able to get a really big competitive advantage to make it worthwhile.

Secondly theres a large ecosystem of add on components designed work with ARM and be dropped into ARM SOCS, such as GPUs, Wifi modules, Gyroscopes, wireless modems, GPS, etc.

Thirdly there are a lot of SOC engineers very familiar with ARM. You can hire them streight from competitors, or college, including PHDs that have done research on it. Youd need to train up any new hirs from scratch on your architecture.

Finally theres a huge software development tool chain built around the established processor architectures. To support a new architecture youd also need to build a set of compiler back ends, bearing in mind the existing ones benefit from many years of tweaking and optimization.