Because people take advantage of your kindness and leave you feeling used.
"If I don't exploit this person's kindness now, I'll fall behind those who do and they'll use that leverage against me" gives you some idea
You only need to hang around toddlers or teens for less than a day to realize people do not start off kind.
People start off egocentric. Unaware or unable to take in to account the people around them are individuals with conflicting wants to you. Also unaware that we are egocentric BUT with social instinct built in to us: if we are surrounded by miserable people, or people angry at us, we don't feel good either.
So we learn that kindness, while sometimes initially painful or less opportunistic, in the long term leads to satisfaction.
It’s not about not protecting yourself against abuse but rather not taking advantage of people.
Being kind doesn’t mean you can’t compete or strategize but rather don’t cheat if you do.
Compassion and acts of charity is kindness.
Stoicism promote exactly this virtue of understanding that you are in control of interpreting your own feelings.
Very easy to over-extend stoicism to your own detriment, physically and mentally.
Kind people always get taken advantage of at work. Others take credit and then left abandoned once there's no more value to the company. I guess that's just capitalism.
That said, don't think that just because you (try to) have few bosses that there isn't some form of hierarchy in which people don't take credit for other people's work.
Sure, maybe there's no boss by title that people suck up to and take credit for stuff to look good to them. But there very definitely will be the "alphas" in the group that everyone looks up to and wants to look good to and the taking credit for stuff will be done to impress those people.
So, if you weed out this kind of stuff successfully well enough, again, I commend you. But I doubt it's as complete as you may want to think. It's just a different looking game of favours and sucking up to with less easily visible (can't just look at title to figure out who to suck up to) lines.
For some people this will be positive as they're good at figuring out who to suck up to in that situation while others may need the title to figure that out. I bet many socially awkward / socially less aware people find it easier to navigate titles they can read in an org chart than sniffing these out of the "sociosphere".