they've done that.Tried it waaaay back in the day when I often didn't see more than one or two other cyclists on my London commute. Hated it, CM, then.
Tried it a couple of years back now millions cycle everyday in London. Still hated CM.
Got caught in one earlier this year. Still hate CM does nowt and achieves even less.
And as for a Critical Mass? Well it hasn't achieved one really in London has it. The bomb dodgers and the BB's might have done though.
If CM, and London cyclists in general, really wanted things to change in their favour they'd organise two flash mob's each and every time a London Cyclist is killed and have two simultaneous die-in's that last an hour at the site of the death and on Parliament Square/Whitehall. And really mess up the traffic in central and really inconvenience others. In time for the evening news. On which an articulate PR person could present the cyclists "Stop the Murder" case. Then the authorities might take CM seriously rather than patting them on the head like small children who make too much noise. Holding a party on a Friday night, yep, that will change the world.
I've been on several, although not for a few years. The idea is a bit tired now that there's a critical mass on CS7 every weekday morning. I don't think people were antagonised (taxi drivers excluded). It was just one big party. There was nothing not to like, other than bad dress sense and too many fixies.