Do you mean treasury stock?

If so,
the reason is according to GAAP,
the entity won't count any gain for that.

It is just a rule.
I hope this helps.

中国各上市银行的股票应该不属于库存股吧，但是我看到很多文献在研究时也将该类金融股删除！

中国各上市银行的股票应该不属于库存股吧，但是我看到很多文献在研究时也将该类金融股删除！

it doesn't mean treasury stock but the companies in the industry of finance
From my perspective, the definition for many accounts of financial companies that are so different from others, the assets and liabilities are mainly intangible, causing different profit earning channels and methods. It's better to talk about the issue for these companies and those in other business segments separately when doing research.
Hopefully that can help.