Adding dual motors is a way to boost profit, as shown by Tesla. Many consumers value the added range, power, and traction that the dual motor version of the S provided more than the price premium Tesla put on that option.
Looks like the Model 3 has dual motor variants now too. One is marketed as AWD, and the other is "performance".
There's a table 1/3rd down this page that shows the different specs. Apparently there's no range to be added by dual motors in the 3 since the single motor version already utilizes the efficient permanent magnet motor. It's a speed and traction boost only.
https://en.wikipedia.org/wiki/Tesla_Model_3