Because they're using energy making an enzyme that only produces a positive impact on fitness in the event plastic is the best food source in the area. In all other cases, a leaner genome that isn't wasting energy on a useless enzyme is reproducing faster.
Even if plastic is heavily concentrated in the environment, if plastic isn't very energetically favorable to decompose, the environment will ALSO need to have a dearth of other available resources in order for the enzyme to provide a fitness benefit.
A good example of the situation is cellulose. Various animals can eat it. Yet trees continue to flourish for hundreds of millions of years since biological machinery to digest cellulose is very energetically expensive and slow.
That's a really bad example as plastics don't have defense mechanisms other than how hard they are to digest compared to other materials in the environment. Trees are alive and have multiple defenses.
