Comment by safety1st
In Deep Work, Cal Newport posits that even the most disciplined, high performers can do work that requires really focused attention for a max of four hours per day. He's a computer science professor, not exactly "management."
And these days, for a lot of knowledge workers there's a pretty strong case that anything which isn't this "deep work" can probably be automated.
So yeah if I'm paying you a full time salary I want those four hours. Without necessarily rendering judgment on what a moonlighting clause should or shouldn't look like, if I'm not getting those four hours, I don't want you on my payroll.
And you think you're more likely to get those four hours in an open office environment with distractions aplenty, as opposed to my effectively noise-proofed home office where I can actually focus?