why guru gobind singh don't have their own bani in guru granth sahib g?

Only Guru Gobind Singh Sahib can answer your question. There is lot of controversy about the so called Baani of Guru Gobind Singh Sahib. You can find considerable material, on the subject, on the internet and YouTube.

It is suggested that you may please click, on the link below, to understand, as to how the Sikhi of Guru Granth Sahib, which teaches spirituality, has been hijacked by Udasis and Nirmalas, over a period of 1718 to 1925 (207 years),:

Hijacking-of-Sikhi_1-to-3.pdf https://share.google/QCMNLMMIhFTZUUUzc