One - you are giving humans and plastic a lot of benefit of doubt. Time and again, it is demonstrated that human convenience trumps environmental concerns. So governments can have state-of-the-art recycling facilities but getting people to co-operate is an uphill challenge.
Second - the nature of plastic itself. You are basing sustainability on cost to environment in production but not taking into account the cost of disposal. The world is rapidly moving towards sustainable sources of energy and thus energy consumption is becoming less of a concern. Glass, cucumbers etc can be sustainably disposed off or recycled but plastic can't be - at least not all kinds currently in use/production.
And the cucumber example is a little disingenuous. A lot of the cucumber product is fast moving and fly off the shelves rapidly. Number of cucumbers being actually consumed is >>> than the number of cucumbers being spoiled. But the environment has to deal with sum total of all the plastic used to wrap the cucumber - even those which were consumed rapidly and would have not been spoiled at all.