It does indicate the “fuel efficient” route pretty clearly though, and always gives multiple other options including the quickest one that isn’t as efficient. If this is what’s causing the issue, OP just needs to look closer at what’s on their screen.
That’s fair, although I think that depends a lot on the type of car you drive. There’s an option to tell Maps what type of car you drive (electric, hybrid, or gas), which will change the results, because cars with regenerative breaking often get better “city” milage than “highway” milage.
It also probably depends on factors like how aerodynamic your vehicle is, because it makes a huge difference above ~50mph (air resistance/drag increases exponentially with speed)