If I understand your question correctly, you are asking why it is not mutually beneficial for someone to give up something of value and get nothing, or something of less value, in return. If that is indeed your question, then you do not understand the definition of mutually beneficial.
If I do $10 worth of work at your home, say cutting your front lawn, and you give me $10 for my work, that is a mutually beneficial exchange. I got what I wanted, $10, and you got what you wanted, a fresh cut lawn.
If I do not work at your home, but you give me $10 anyway, and you get nothing in return for your money, that is not a mutually beneficial exchange.
There is the possibility that you derive some spiritual or emotional benefit to giving money away, in which case you would see giving me $10 and getting nothing in return as mutually beneficial, giving your money away made you feel better about yourself and you value that feeling more than you valued the $10.
However, once force is added to the equation, it is tyrannical for you to force people to sacrifice value against their will:
I show up at your house. I do no work. I demand $10. Whether or not you are willing to give me the money, Mare and Zen come out of the bushes and threaten to violate your rights unless you hand over $10. Now you are being robbed. It doesn't matter if you would have given me the money anyway, you no longer have a choice.