You can have an adaptive dictionary, favoring recently seen entries. The dictionary would have to be rebuilt periodically in practice, but it has an advantage o

You can have an adaptive dictionary, favoring recently seen entries. The dictionary would have to be rebuilt periodically in practice, but it has an advantage of being able to cope with the ever-changing ("non-stationary") data distribution. Sometimes the dictionary doesn't even need to be stored and instead can be calculated as the data gets decompressed; this is so big deal that many compression algorithms indeed omit and adaptively build the dictionary instead.