Bio degradable doesn't mean that it will degrade on its own because some bacteria ate it. For example, PLA is bio degradable, but you need an industrial hot composter to turn it back into lactic acid. You can't put it into a waste bin with your banana peel and expect it to degrade, that won't happen.

The problem with many bio degradable plastics is that they also degrade either due to heat exposure (PLA will start softening at around 55°) or due to UV exposure (PLA will fall apart within months to a year when exposed to the sun) or both (like PLA). That means their use is limited. You don't want your glasses to fall apart after a year of use, do you? But if you can use such plastics away from the sun, water, heat, etc, they can last forever.