They may spend it on drugs, alcohol, or gambling
Giving money to beggars is wrong because they may easily spend it to perpetuate a cycle of abuse or addiction. The giver has then contributed to a negative cycle.
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A disproportionately large percentage of homeless people are substance abusers. The Substance Abuse and Mental Health Services Administration estimated in 2011 that on a given night, 34.7% of homeless residents in a shelter were chronic substance abusers. Substance abuse is often a perpetuating factor of homelessness. Furthermore, addictive disorders amongst the homeless tend to be much higher than the general population. The Nordic Studies on Alcohol and Drugs found that gambling was prevalent in 11% of homeless people studied, compared to 0.7% of the general population.
If you are in a financially stable state and can give to the poor, you have the responsibility to ensure your money does not harm or perpetuate harm for another person. Statistics show that people who are homeless are more likely to struggle with substance abuse and gambling. It is better to be cautious with your money than to give it away to someone who is statistically more likely to use it for negative purposes.  There are other ways to help a homeless person rather than giving money. A donor can give a gift card, offer a ride, offer to fill a tank of gas, or food. Giving items ensures that the person in need will not use cash for drugs or other negative things. Homelessness cannot be cured by one person giving another person money. Homelessness and poverty must be solved more directly by other ways, such as giving to charities that know what beggars really need.
For every homeless person who is a substance abuser, this is another who is not . You do not know what they are going to do with the money, so you should not assume that it will be spent on a bad habit. Not giving money at all erases the chance that the money you give will be spent productively.
People become homeless because they are irresponsible and turn to drugs, alcohol, or gambling; they cannot be trusted with money.
[P1] People who are homeless are statistically more likely to use cash for drugs, alcohol, or gambling. [P2] You earned your money fairly, so you have a responsibility to ensure the money is used fairly or positively.
Rejecting the premises
Everyone should work hard to make their own lives better rather than depending on the goodwill of others. It should not matter to you how a homeless person would spend your money, because you should not give away your own hard earned money in the first place.