Historically, married filing separately brackets were exactly half of married filing jointly and this continues to be the case almost all of the time.
Historically, this generally produced higher taxes for people filing married filing separately than for people filing as single with the same income.
Since people generally file married filing separately to gain some tax advantage or are in that situation temporarily due to non-cooperation of the parties in a pending divorce, this wasn't very concerning to most policy makers at the time. The historical married filing jointly rates were set so that roughly half of married people received a marriage bonus, while roughly half received a marriage penalty.
Subsequently, this was reformed to eliminate the marriage penalty that married couples who file jointly experience relative to two single people filing, when their incomes are very nearly equal. (Most married couples have very unequal incomes and in those cases, married couples pay significantly lower taxes than they would if each of them separately filed as single and the reform, in general, benefitted married couples whether or not their incomes were similar and caused them to experience a "marriage penalty.")
While most of the tax brackets were reformed to eliminate the "marriage penalty", the statute enacting the 37% income tax bracket, passed in December of 2017 and effective in 2018, was not among them.