Why do some people say real estate taxes are taxes on middle class people?

Middle class people buy their own houses, and own them, and pay taxes on them. Poor people rent houses or apartments. (There's a lot of overlap and gray area between these categories!)

Rich people have their homes attached to various trusts and sometimes charities, and these are often taxed at a lower rate, or the taxes are paid as a business expense. So the wealthy people often don't pay their own taxes; they have somebody else to manage the business of minimizing their taxes.