Software design is different from hardware. Software is less wrecked by having multiple simultaneous authors.
Software is mostly verified by testing and iteration, and is usually shipped with lots of bugs anyhow. Software bugs are quickly fixable: hack the code and push out an update. Version 123.17.91b or something.
Hardware takes a lot longer to revise and to implement updates in the field. Much more expensive too. So it's better to have one really good person be in charge and responsible.
The difference is compounded by the trend of having armies of reasonably skilled programmers around, and precious few decent circuit designers.