system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: No idea if it's crappy design or malicious design, but I've noticed that packaging never wants to tear open where the perforated line is. Why is that?
gpt
Most likely the blade that makes the perforations is either worn out or not adjusted properly, so it just marks the packaging a bit instead of significantly weakening it. 

To fix it, the manufacturer would have to regularly take a sample and see if it tears easily where it's supposed to, then stop the production line to fix it whenever it gets close to failing. Stopping production lines is expensive, and QA costs money too.