Source? It would be surprising to me if lenders were regularly approving loans where the mortgage payment alone (not including insurance or taxes) was >=48% of monthly income. There are certainly lenders that will approve mortgages with high ratios relative to salary (due to the value of bonuses and equity comp), but the $125,000 figure is all-in income, so this isn't relevant.
A 40% ratio for the $125,000 household would give you a maximum purchase price of $830,000, which would be a conforming loan in the Bay Area, not a jumbo one.
In the South Bay near Mountain View, there's basically no single-family homes under $1.5M, and the majority of houses go for $2M up. Many are in the $4M+ range. You can get cheaper in San Jose ($800K-ish for a run-down 2BR from the 50s) or Daly City (low millions), but most of the Peninsula and SF has a floor at a million.
Most Bay Area residents just don't own homes. They rent permanently or maybe own a condo. But the context for this sub-thread is "people with a house", and basically all mortgages for single-family homes are jumbo if you're not paying in cash or putting >50% down.
The median Bay Area household cannot afford to buy a house, yes, but that does not mean that those buying houses are significantly wealthier. The Bay is far, far richer than you can imagine.
Buying a house for $1M is firmly middle class status. True lower class people simply cannot afford to live in the Bay at all.
To be in the top 1% in the Bay Area you have to make close $1M per year.
A 40% ratio for the $125,000 household would give you a maximum purchase price of $830,000, which would be a conforming loan in the Bay Area, not a jumbo one.