You're forgetting about FICA/Social Security which can be as high as 12.4% of AGI for every dollar earned below $150k or thereabouts. A stay-at-home spouse doesn't pay social security but nonetheless benefits from it (and in certain states, can do so tax free) upon retirement or death of the breadwinner. That's not a discount available to an individual taxpayer or a double-income household.
Filing as married allows for higher tax exemptions, in many cases around double the individual tax exemptions. For a high earning breadwinner this is as much as twice the "normal" buffer of untaxed income for investment before incurring a phase-out. If both spouses were high earning there wouldn't be much of a difference between the average exemption per spouse and the exemption provided to an individual taxpayer. Sometimes it's even less than if each spouse had filed seperately.
Filing as married allows for higher tax exemptions, in many cases around double the individual tax exemptions. For a high earning breadwinner this is as much as twice the "normal" buffer of untaxed income for investment before incurring a phase-out. If both spouses were high earning there wouldn't be much of a difference between the average exemption per spouse and the exemption provided to an individual taxpayer. Sometimes it's even less than if each spouse had filed seperately.