I'd say there's lots of reasons why; they're an offshoot of XR that have a specific strategy that doesn't involve those type of actions, maybe they think those type of actions will be politically damaging to the cause in the long run, those actions take a huge personal risk that maybe JSO people aren't willing to take, society has a tightly policed set of type of activities as 'acceptable' dissent, etc. They also lack lots of the avenues like strike action, workplace sabotage, etc. that other political movements have had.
There's a load of things happening that are horrendous, and plenty often don't result in actions like you mention for complex reasons.
Not saying I think JSO are right, but I can understand why they're doing the type of things they are.