我哥係香港住左18年，佢衣家19，但點解張身分證唔係永久，而係「香港身分證」？？？？

It depends on the exact reason why your brother stays in Hong Kong.

Immigration Department practically issues ID cards to all people who hold any extended stay visas (student, work, etc.)

Their terms of staying are different based on their qualification. This qualification will determine if they qualify for permanent residence in Hong Kong or not.

As there is no information provided, your question can't be answered.

若果是非香港出生的話，永久性居民身份是不會自動取得的，需向入境處提出申請。

只要合資格 , 可核實永久性居民身份證資格:
http://www.immd.gov.hk/pdforms/rop145a.pdf

合資格可上http://www.immd.gov.hk/hkt/forms/forms/rop1.html下 載 表 格 再遞交 (香 港 永 久 性 居 民 身 份 證 / 香 港 居 民 身 份 證 申 請 書)