On reason that's mentioned in the article: paranoia. If the chip is open-source, you can at least theoretically audit its manufacturing to make sure no backdoor is being inserted.

Also, return to the taxpayer. To me, a publicly-funded verification of RISC-V would bring wider benefits than that of a proprietary ISA like ARM [1].

http://www.cl.cam.ac.uk/~acjf3/arm/