I'm trying to figure out why infrastructure costs so much and is so backlogged.

Seems like there are no simple answers. 

While Roman buildings made from concrete are doing fine, we build ours with rebar that rusts of 50 years or so and then causes the concrete to fail.  Most of the infrastructure was put in in the 50's and 60's.  So it's all failing now. 

Do they make that short sighted decision to save costs?

Or because they thought infrastructure technology would vastly improve over 50 years?

They do it because of earthquake resistance I think.  So what makes more sense, build a potentially timeless object and hope that an earthquake doesn't take it out... or build one that is -- in the terms one should probably think about this in -- more or less "disposable" so that if an earthquake happens during it's tiny lifespan that we'll all be safer? To me, the latter, which has been the American strategy from the latter half of 20th century, seems like cutting off one's nose to spite one's face.

One thing they are doing now is suggesting a code requirement for galvanized steel rebar ,which would add a whopping 20 years or so to the lifespan.  Right now, much of the rebar is not galvanized and in fact may be starting to rust before the concrete is poured, which shortens the lifespan.

BTW I'm getting all this from what I heard on NPR the other day, not the linked video.

I was wondering where you were getting that from. Thank you for the information!

The question remains why something so "disposable" cannot be made cheaper so it's more regularly replaced. 

Labor in ancient Rome was "cost effective".

So the main cost of infrastructure now is labor? I thought materials are significant too.