1/ Assuming (and this is a big assumption) that the lockdown governors understand the lack of evidence that masks are useful - especially outdoors - why would they insist on them? Three possible reasons come to mind, one semi-legitimate and the other two uglier...

2/ The semi-legitimate one: making everyone wear masks theoretically lessens the stigma for sick people to wear them, and thus makes compliance by the sick more likely.

The ugly one: masks give governors who want to end lockdowns an excuse - look, we figured out masks work.

3/ The really ugly one: masks are a form of social control, and the lockdown governors have learned to love social control; they are an easy way to shame anyone who does not agree that #COVID is very, very dangerous, and very special rules are needed...

4/ As an added bonus, to the extent masks work at all, one reason may be that under normal circumstances they DO signal sick people, and thus encourage healthy people to remain at a distance; requiring them to be worn universally ruins that signal and thus makes them less useful.