A lot of water infrastructure is physically spread out. It be very expensive and cumbersome (and probably inefficient) to require staff to by physically present at each site for monitoring and making any changes.
No such thing. There are levels of security, where higher levels mean more cost. And even with the most secure facility, there are no guarantees. And certainly we don't have the funding to provide that level of security for every single facility.
A well designed and deployed VPN link from the satellite location back to HQ (to machines isolated for just this use) would be orders of magnitude safer then what they seem to have been doing, and a reasonable compromise between security and utility.
Its not like we lack the labor to do it. Plus how many years or decades of wages for someone to stand around would be required to exceed the cost of infastructure going down even just a few days?
I mean first you need to figure out the cost of a large municipality getting stuxnetted, and then you need to figure out where to hide when your constituents find out you made that kind of call.
