Why is the plural form of "life" "lives", while the plural form of "still life" is "still lifes"?
Solution 1:
I would say still life has undergone reification, which transforms it into a "standalone word". How the subcomponent elements work grammatically doesn't automatically affect how the composite form works.
Effectively it's a kind of neologism - not really "new" today, but a lot later than the original word life with its irregular plural. Neologisms almost always have regular plural and verb forms.
Solution 2:
Irregular plurals and irregular deverbal nouns (i.e., nouns formed from verbs, like house /hauz/ from house /haus/) are frequently restricted in the way that your example and Barrie's (computer) mouses illustrate: the irregular form is only used for the most common meaning, with a fully regular form being used in neologisms and other variants.
Other examples are:
I might wonder when my computer was hard-drived, but not hard-driven.
If I hit someone with the sheath of a sword, I might be said to shea[θ] them, but not to shea[ð] them (which only means "to put a sword in its sheath").
When the evil witch gets squashed with a house in The Wizard of Oz, she might be said to get hou[s]ed, but not hou[z]ed (which only means "to provide with housing").
When bad measures street cred, its superlative is baddest, not worst.
The past tense of input is, for many people, inputted (as in Who inputted these data?), even though the past tense of put is just put.
Solution 3:
Because still lifes are not the same thing as still lives. Similarly, mouses are not the same thing as mice.