They do and they also know the old Sir Humphrey quote - a Bold decision could cost you your seat, a BRAVE decision could cost you the next election
Oddly this is OK - of all the issues facing Govt a new London runway is probably 99th out of 100 in importance (I know most people on here disagree but this IS a flying forum)
if it isn't built we'll all find a way round it - larger aircraft using the slots at LHR & LGW, more use of the Chunned to near-Europe, more flights from Stansted, Birmingham, Manchester, Newcastle etc, more interconnections in Dubai, AMS etc etc
The overall cost may be higher but it will be spread over a much longer period and more people