DISCLAIMER: I'm not a linguist and I don't have an authoritative source to back my reasoning, so take my answer with a grain of salt.

[Grammatical case][1] reflects the grammatical function performed by a word in a phrase, clause or sentence. Noun in a given case (e.g. *úi-ge*) remains a noun and retains its meaning.

That gives us multiple good reasons to not treat the word-forming suffixes, like *-siz* as additional grammatical cases:

 - suffix changes the part of speech, *úi-siz* becomes an adjective (as @AdamBittlingmayer said in the comments);
 - suffix doesn't reflect the grammatical function of a word in a sentence;
 - suffix creates a new word, with a new meaning (as @kabraxis said in the comments);
 - this new word, can have a grammatical case of its own:

    1. nominative *úi-siz* - homeless  
    2. genitive *úı-siz-diń* - of homeless  
    3. dative *úı-siz-ge* - to homeless  
    4. accusative *úı-siz-di* - homeless  
    5. locative *úı-siz-de* - by (at?) homeless  
    6. ablative *úı-siz-den* - from homeless  
    7. instrumental *úı-siz-ben* - with homeless  

It's much more convenient to treat *úi-siz* as a new word than a case of *úi*.


  [1]: https://en.wikipedia.org/wiki/Grammatical_case