For location, it’s balancing competing interests again, spiced with the excitement of trying to see 50 years into the future.
A 2m pipe takes up a lot of room and very rarely gets dug up. Roads are the easy place to put them. Otherwise they tend to end up under buildings as development goes on. Alternatively, you would need massive setbacks from the road to businesses and homes. People also like to do things like build basements which are generally deeper than water and sewer lines. Water and sewer are generally 2.5m down to minimize freezing issues in winter. My basement goes down 3m and there’s a sump below that. Bigger buildings with multi-story parkades can go seriously deep. As a result, a lot of utilities, which should rarely need excavation, go under the road.
Subways often run along roads for similar reasons. Vancouver is expanding their subway (sky train), and it mostly follows roads because its cheaper and easier to dig down and burry it than to bore tunnels (see Toronto’s nightmare with stuck boring equipment).