Apparently Mercator wasn't the original projection they used[1]. But they found that it worked better for giving people directions because it preserved route angles in higher latitude regions (it's benefit for navigation is the reason it became so popular, from my understanding).

For what it's worth, I feel like I don't actually come across the Mercator projection that often (I'd say the Robinson projection is more popular in schools, for instance).

[1] https://www.theverge.com/2018/8/5/17653122/google-maps-updat...