The people were naive. It's not like they had malicious intent. 'Don't hate lazy people - they did nothing'
Manslaughter is a criminal charge. As is criminal negligence.
And quite apart from facing criminal charges, the folks who introduced lead in the gasoline, produced microplastics and PFAS, and more all have the kind of wealth that gives them raw economic and political power and with it, the ability to do continued harm.
See also the Cuyahoga catching on fire. Water burning is quite a poster child.
It is worth noting that this probably had more to do with the awful state of the city sewage system, so the water was well known to be contaminated. Luckily we've improved somewhat with sewage (I think).
Although anecdotally I've had a local lake closed for weeks at least due to toxic red algae blooms related to run off from farms (I believe it's fertilizer runoff that's attributed to these blooms)