The tax rates on a single combined income are higher than the tax rates for the two uncombined incomes.

Tax rates aren't the same for all tax brackets. So by not being married, despite having the same actual combined income, taxes are lower because it's taxed as two lower income brackets rather than the combined higher one with a higher tax rate.

That isn't exactly true... married filed jointly rates are exactly twice as much as filing separately in the United States, unless your combined income is more than $622,050.. so you are only correct if you make more than that.

https://www.bankrate.com/finance/taxes/tax-brackets.aspx

Also, it sounds like you might be confused about how income tax brackets work. Your income is not taxed at one rate... each amount is taxed at the rate for that bracket. This means that EVERYONE pays 10% on the first $9,875 they make, no matter how much they make in total. This is how marginal taxes work:

https://www.investopedia.com/terms/m/marginaltaxrate.asp

I don't live in the united states.

The comment you were replying to was stating how it works in the US, and your reply made it sound like you were talking about the US still.

I get now that it was part of the conversation about the differences between the US and Canada.