Didn't we try this same argument already with gun manufacturers? How well did that work?

The problem with holding manufacturers responsible for releasing a potentially dangerous product is that the manufacturers view compliance with regulation as an obstacle to maximizing profits, and seek to avoid it. Manufacturing is also centralized and they organize and lobby politicians to influence regulation.

Consumers are distributed and disorganized, and I predict that blame for not securing your IOT device(s) will stay with consumers for the foreseeable future.

I agree some standardized testing would be a good idea though.