It's completely practical and deployed globally. But knowing there's a leak (akin to "knowing there's a bug") is not enough to find and fix it. And the cost-effectiveness drops to likely negative for a small place.
A few dozen sensors can go a long way, for example a municipality can find that a particular pipe is consistently pushing 100gpm, despite serving residential customers that don't normally have that kind of even usage. This implys a leak on that section of pipe, making a root cause analysis (and shutting off just the worst leaks in the system, rather than the whole town) much easier to do.