Why does a plastic lunchbox tend to be more greasy than glass one?
There are few reasons why this happens. 


Plastic get dull much faster than glass. Just one wash with sponge make the difference. Making more surface oil can stick to and harder for detergents to go in grooves and wash it away.  
Oil reacts with plastic as both are (In very very layman terms) the same. Plastic is made from oil. So when you have warm (or even hot) oil put into container (or warmed up in that container for example in microwave) you start reaction called polymerisation. The same that you use to create non-stick surface on cast iron pans.
But here the bonds from oil bond to plastic. So you make the plastic surface oily and non-stick forever. The better quality containers have this process made during production so the polymers will "fall off". 


To avoid this: 


Don't put hot food in plastic container. 
Don't heat up food in plastic containers. Especially ones that have dying agents (famous "tupperware after one turmeric") 
Don't wash them with rough sponge. Usually a little bit of dish soap and hand is enough. Dishwasher might be safe from temperature point but the container is moving in the rack.