Because of zoning and anti-development policies by the government.

Some people making $15/hour will get lucky and get subsidized or “affordable” housing though and be able to live on that. Or they’re living with their parents.

Maybe if you define major as Seattle, New York, or other similarly expensive cities. Places like Jacksonville FL or Columbus OH, both of which are larger and more populous than San Francisco, $15/hr is pretty reasonable for a job with minimal education or experience requirements. A household with two income earners will spend less than a week's combined income on an apartment for example (which is easily under $1000/month either place).

What do you mean unreasonable? Does a blanket statement like this really apply to every major US city? I see multiple apartments for $800 or less in LA, SF and Seattle on an apartment search site.