Why aren't plastics biodegradeable?

We put this to John Williams, Polymers and Materials Manager, at the National Non-Food Crops Centre:John - Well, I mean, traditionally, if you look at the materials we've produced from petrochemicals then the majority of plastics need to actually be durable. That's what they've been developed for - maximum functionality. And there are one or two pressure of petro-derived plastics that are biodegradable very used in speciality sort of type situations like medical applications and so on. What's happened recently, of course, is that the drive toward some aspect of biodegradabilities as a function is quite useful if you look into co-disposal of things which are also biodegradable like food waste, for example. So, the answer is you actually you - the important thing is you pick the material for its functionality in terms of what its use is going to be rather what its disposal route is going to be. But now, at the same time you say, actually if we did say, develop a biodegradable version of that, is that useful way to go or not?