Globally, because encoding and decoding are not the same thing. Encoding requires making choices about what to throw away, but decoding is just putting them ba

Globally, because encoding and decoding are not the same thing. Encoding requires making choices about what to throw away, but decoding is just putting them back together. Mathematically you can think about encoding as converting to some matrix representation that has a lot of zeros -- the process to take that matrix and reconstruct the image doesn't depend on how many zeros there are, but the efficiency of the encoder does. Perhaps an easier way to think about it from a programmer perspective.. a decoder is a programming language interpreter that takes a program and generates the data. The encoder is something that generates a program in that language. So you can see that the decoder is quite straight-forward, while the encoder has a lot of choices to make about how best to represent the data as a program in the decoder's language.