Simplifying the whole screening process (many components and dependencies are involved), the 30dB attenuation at 1GHz reduces the radiation exposure by about 1000 times. It is a very high and completely satisfactory result for this type of products. Raising this value does not have more significance. T-shirts or blankets are designed to reduce the impact of radiation. If we wanted to completely exclude this radiation, we would have to create very tight suits, such the ones used by servicers of broadcast antennas. Then those 80dB would really have some meaning. These employees, however, are in contact with much higher radiation impact and need another kind of protection.
To illustrate better this problem, think of a leaky vessel. The vessel will be seeping water no matter how much we will modify the density (permeability) of the material from which it is made.