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 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

• suffixes can be combined multiple times, creating new words: úi-siz-dei - like a homeless, úi-siz-der-siz - without homeless.

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

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 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 doesn't reflect the grammatical function of a word in a sentence;

• suffix changes the part of speech, úi-siz becomes an adjective (as @AdamBittlingmayer said in the comments);

• 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

• suffixes can be combined multiple times, creating new words: úi-siz-dei - like a homeless, úi-siz-der-siz - without homeless.

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

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 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.

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 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

• suffixes can be combined multiple times, creating new words: úi-siz-dei - like a homeless, úi-siz-der-siz - without homeless.

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