Two main problems
1. Middle and senior management who don't want to lose control or be rendered less effective.
2. Engineers who are not trained in written communication and largely cannot autonomously move a group towards a goal without a lot of supervision.
If you solve for no 2, then that acts counter to no 1 - because middle management will be questioned - why do we need you ? If a group of engineers can function on their own towards a common goal, then the manager's role is more or less rendered redundant. Sure there may be a need for psychological support but you surely won't need the current ratio of engineers: managers.
There is a deep rooted old school interest in staying physically connected. This won't go away anytime soon. I am not debating whether that is right or wrong, but the general notion that 'we are better if we are physically together' still persists. I don't know if this is a genuine feel-good-together feeling or just a made up emotion to mask point no 1 above.
I am flummoxed by how executive leadership is simply blind to these facts in most companies. I mean the CEO can declare a fully remote constraint sort of like the exact opposite of what Musk did at Twitter and drive productivity higher. The cynic in me says execs can't force this decision because the senior management simply will come back and say 'we cannot be this productive with a fully remote team anymore'. I don't know but I for one cannot understand the irrational exuberance behind RTO.