Also, for some developers, it simply doesn't matter. Rodinal is packed in HDPE and it gets plenty brown in the container...likely oxidizing yet it works great. I'm sure when chemical manufacturers are looking at their bottle choice they consider compatibility (O2 ingress, chemical compatibility) vs shelf life. If HDPE works and they are looking for a 2 year shelf life, they'll use the HDPE and save a dime a bottle.