You could say why... but instead, you left it to my imagination.
a) I'm unconvinced that my money will make a positive difference.
b) I don't want to reward begging.
c) I don't find interacting with the homeless enjoyable.
d) If I give the quarter to the same homeless guy each day he'll come to expect it and become unhappy if I don't one day.
e) Doing the same thing every day quickly makes it routine and destroys any enjoyment.
I'm just surprised that people always use the "giving money to the homeless" thing as an example of doing good, when there's far better stuff around. (For example, you could save your $7.50 for twelve months[1] and mail off a $90 donation to the charity of your choice).
[1] If you give less frequently you'll minimize the cost of processing your donations.