Why do Muslims give a fixed amount of money to charity?

Answer 1

because it is the minimum ammount you can give, for example if you have loads of money, you can give as much as you want, but if you are just coping but you still want to give money to charity you have to give the minimum amount atleast

Answer 2

It is an alms giving Islamic ritual worship that requires paying 2.5% annually to the poor and needy people, called Zakat.

Note: This is only a Sunni practice. The Shiites are supposed give 10% annually to Zakat.