Those 4 bytes have barely consumed over a megabyte in the 300k+ blocks we've mined.
It's copied and stored a million times, why not use the space more economically.
Because this entails:
- The consumption of some development resources.
- A more complicated protocol.
- A greater potential for bugs.
The costs of dealing with this problem dwarf the potential gain of sparing all nodes 21MB/century of storage.
- Are developer resources busy arguing about the lack of capacity on each block?
- The protocol will be simpler because there is one field less or what am I missing?
- Less complexity reduces the potential for bugs, the implementation, as every change, presents a possibility for error.