I've seen what you're talking about, in practice, many times. There was a blanket organisation in Australia that attempted to unite activists. The first onrush of enthusiasm was relatively enormous. There was was one thing though that united everyone, their contempt for the Prime Minister. The signs at the nationally 100 000 rallies (in a country of 28 million that's a significant number) were for all the different causes and issues the then Prime Minister was a threat to. So... after the media bias and lack of immediate change, people went back to focusing on their own issues. I had an umbrella, still do, but people just don't want to band together. For example, asylum seeker advocates have the environment much further down on their list than environmental advocates, and vice versa. Only a few see a common ground, yet for a broader thinker, there is one.
It upsets me that you are right, but I know it to be true.
There is hope though, and we have to keep that attitude.
If your roof has 10 holes in it and you fix one, then another one later, while another hole is appearing, then it's still better than not fixing them at all. That is the only thing that stops me from literally climbing onto a rooftop and using a megaphone to call everyone idiots, and as you say, I don't know what each individual does in their private life. I may be speaking to someone who rightfully believes that I am complacent idiot compared to them.
And then there's the "anti-hatred" activists that protest protests and start swinging at the haters. (and gets global news coverage doing so, and harms the reputation of any movement they are involved in)
Save the forest signs on brand new pieces of painted cardboard. Getting to your gig to play your anti fossil fuel , anti deforestation song, in a van, on a road. Many people seem to forget the privileges that are a direct result of what they are striving to change and balk at being confronted.

Buddhism 101, create change from within first. Then you can start saving the world.