Why don’t dictionaries have all the different forms of a word

It's not usually necessary to list all forms of a word formed by adding common prefixes and suffixes. These affixes have common meanings in the way that they modify the root word, so unless the modified form has an ideosynctatic meaning, most dictionaries won't bother describing them explicitly. If they did, practically every definition would go on for many pages, because English allows affixes to be added in an almost unlimited manner, and this would require more lexicographers whose only task was to write all these tedious, obvious definitions.

That's how you get words like "antidisestablishmentarianism"; the root word is "establish", and two prefixes and three or four suffixes have been added (I'm not sure if "arian" is one suffix or the combination or "ary" and "an"). As each affix is added, the meaning is altered in the usually expected way, so you can understand the final word just from the basic grammar.