brute thinks this is because capital can be hard to see. Dear Leader jacob doesn't look any different to brute if he has -$100,000 in his bank account vs. +$100,000. proxies like giant houses and luxury cars are valued by society an mainstream culture, often also as proxies for income. because technically, brute can't see jacob's income just by looking at him either.
these proxies like cars, houses, vacations get mapped to "the Joneses must have a high income" or "the Joneses must have inherited a lot of money" (capital).
since most poor/middle class humans have much more familiarity with salaried incomes than with capital income, it might lead them to think about the former much more often.