polylactic acid degradation isn't for the timescale of years though; it just has to beat decades to centuries.PLA isn't really biodegradable though, requires specific compost facilities to process. You can see people's 3D printed junk last for years and years outside in the sun.
Plastic straws are also uniquely flexible, which makes storage and packaging a breeze when compared against alternative which are usually brittle.
Ultimately this fight over packaging is just green washing. There are far more environmentally damaging things that humans do (illegal crude refining in third world countries for example) than some straws.
In environmental conditions the half life of HDPE is 58-1200 years.