They know the second they do something actually
effective, the kid gloves come off and they get crushed. The media can't cover for actual bomb-makers, even if NLG, Antifa, and other fringe allies would support it.
As long as they are using "makeshift tools", they can pretend they're innocent protesters scraping together improvised defenses against the murderous thugs in uniform. It's not true, of course, but it only has to look
plausible to keep the narrative afloat. It's how they enforce the
decision dilemma against organizations with superior firepower.
The second they start using pre-made bombs, they lose the "it was peaceful until the cops escalated into violence" narrative. It's why they call these things "cop riots", as in "we weren't planning on violence but then the cops started some". It's also why they're so desperate to plant the "everything bad is an infiltrator's white supremacist false flag" idea, they know it's only a matter of time until some idiot tries something and they have to shift blame.
We've noted their talk about "bodies" before--that is the language of defenseless people with no ability to fight, and they're using it for that exact reason. Bombings are the tools of organized militant forces, who pose a legitimate threat. They aren't ready to shift into that narrative. It's why the charges of LARPing stick so well, and why you should laugh at every idiot who says they are #Resisting or #revolutionary.
(If they
do shift, then we enter a different phase. The fed response hardens, the public narrative which already doesn't like the riots turns on them, and my "we aren't in boogaloo" assessment changes.)