You can't make a strong explosive in a portable plastic bag. If you get the chemistry right, you will just burn yourself and maybe your neighbor. You won't even lose fingers, that requires better conditions. AFAIK, that's exactly what happened to the original liquid bomber, that was caught after he burned himself mixing things on the bathroom. In much more stable conditions than a bag, but still not nearly stable enough.
The guy that tried to carry solid explosives on his underwear was also caught only after he burned himself, because that's also not a practical way to carry them. As did the guy that tried to carry them on his shoes.
There are some very robust reasons why all those plots are doomed to failure, but those are the ones the US focus on (and basically impose on the rest of the world), while there are many perfectly viable vulnerabilities to exploit that nobody wants to close because they would impose more restrictions on the passengers. And that nobody is exploiting because it requires knowing what to do, and people that know what to do aren't normally prone to killing random strangers.
