Why is taboo to ask about other people's salaries?

I know it's a widespread belief somehow and that's why I don't actively ask others. However, it feels a little weird when it's between close friends.

So me and my three close friends just graduated college and we soon finally got our first jobs. We were always open with each other prior so I feel a little strange since it seems that we all mutually had a silent agreement to dance around the topic of our respective salaries at our jobs.

Why is that? There are times I want to ask and there are also times I want to tell them my salary but whenever our conversation leads to the topic of salary, I myself clam up and quickly get on with what I was saying before and just skip over the salary bit altogether.

Idk, it's just a little frustrating since it feels so secretive for no reason at all. We share everything like our problems, our complaints, and whatever with each other but then suddenly something like salary amount is now becoming like a wall we never had before.

So yeah. Is it really THAT rude to ask about someone's salary? I know some tends to be insecure about a small salary but since we are literally fresh graduates, I feel a small salary isn't that surprising with people our age.

Edit: Grammar