I agree with the latter part, but for the former note that Earth's surface is 2d: you can go around holdouts; unlike roads you don't care about the fiber being straight.
Actually you do. Longer fibers need more repeaters, and the ideal fiber on land is strung down pipes (this allows later capacity upgrades). Both of these are cheaper if existing roads are paralleled.
Sure, you want to optimize for total length. However, you don't have any meaningful restrictions on turn radius, which do exist when you want to build a road and which make building a road around holdouts oftentimes impossible.
Simple, cheap mesh networks based on consumer WiFi hardware are very short range. To get any reasonable distance between nodes you need towers and directional antennas.