[somenameforme]

No idea what the SAT is like now a days (based on the comments in this thread) but 'back in the day' if you knew enough about a problem to understand the math needed to solve it, which is needed to plug in the answer, then it'd generally be faster to just solve than working backwards since the number of steps would typically be less than repeatedly plugging in. And most/all questions also had a 'None of the above' option that was the answer a fair chunk of the time.

Another practical thing is that tests seem to trend substantially higher in difficulty when multiple choice. I was part of the first class at my university that took a calculus program which was multiple choice and we thought it was going to be a cake walk. But suddenly like every single test problem was using obscure trigonometric tricks on top of the basic calculus itself. And of course no partial credit for getting everything 95% right and missing one really disguised trig trick at the end. Grades for the class were significantly lower than prior years, because those tests were just nuts - and I'm a very much a math guy.