Why polyurethanes not rubber?

Most physical and chemical parameters of polyurethanes are few times higher than rubber parameters. Polyurethanes are fully resistant to hydrolysis so they are not rinsed out form the application. Moreover, rubber soaks water (to 1 mm deep a year) and in case of polyurethanes such effect does not take place. What is more, to choose appropriate rubber work conditions of coat must be very precisely analyzed and very often there is necessity of using highly modified mixtures, difficult in processing. In this case polyurethanes are much more universal.