SoftBank is originally one of the big 3 telecom providers in Japan. We do joke around here about our phone bills increasing anytime Masayoshi Son makes a bad investment.
There's a reason why the iphone team was so small (https://www.theregister.com/2014/03/27/apple_employee_reveal...) you need to be ruthlessly focussed and not pressurised by outside investors or managers who don't know what they're talking about (Softbank)
I'm not sure what the money would be spent on at OpenAI. Maybe they will keep it small, and hyper-focussed. But I worked in VC for a while and VC money was described to me as rocket fuel, you don't want to go dousing it on things before the rocket is built
They were an early investor in WeWork, and paid Adam Neumann to go away instead of throwing him out head first.