No respect for privacy: Everyone has his own privacy, including his own salary. Asking about colleagues' salary is an invasion of their privacy and personal information.
It will cause unnecessary jealousy and controversy: if you find that your colleagues earn more than yourself, you may feel inferior and jealous. This may affect the working relationship and atmosphere.
Salary is a sensitive topic: discussing salary may lead to some unnecessary contradictions and disputes, especially some people feel that their salary is not fair enough or their colleagues earn more than themselves. These situations may affect working relationships and teamwork.
In a word, it is inappropriate to ask colleagues about their salary and should be avoided. If you have any questions or dissatisfaction about salary, you'd better communicate directly with your employer or HR instead of asking your colleagues about salary.