The problem is, as usual, one of power imbalance. Employers -- especially those in industries where unions and collective bargaining is unusual -- tend to have a lot more power in the relationship than their employees do, so they get to dictate terms, at least up until the point that labor willing to accept those terms becomes scarce.

I think we can agree that if every software company decided that they were going to be a full-time pairing shop, that'd be a bad thing. In the same way, I don't think it'd be too much of a stretch to also suggest that it'd be bad if all employers decided that their workers had to be butts-in-seats 40 hours (or more) per week, no exceptions.