Because those they vote for promise to make the ‘poor man rich while making a rich man poor’. It will never work.
Unfortunately, their promises "work" to get them elected time and time again. And yet, despite the evidence to the contrary, hoi polloi keep electing them because of ignorance, usually due to persistently poor (or even deliberately perverse) education particularly in areas of history and economics.
Socialism (rich few, working in or with the government, benefiting at the expense of multitude of "equally" poor, with no middle class) doesn't work for anyone except those who are making these "promises".