Could it be that the public transport system at MAN (and the five other
airports) is that bad and isn't an option to the car?
If the public transport is considered as 'bad' at Manchester with such frequency and variety of transports, which is only set to improve with an extra heavy rail platform and metrolink, then transport at other airports must be appalling!
May be down to number of people who drive too? London has a high dependency on its public transport (just look at what happens when the tube goes wrong), because I assume people do not 'need' to drive there (I didn't learn to drive while I lived in London due to transport but did learn when I moved to Cheshire).
I 'need' to drive now as the public transport in my local area is bad, so, while people can take a tube/train/bus to the immediate London airports, the poor public transport
in my local area is poor and gets me no where near an airport.
The transport for intra-Manchester and immediate large towns is excellent, second to none in my opinion, but to get to the smaller towns which fall in the catchment is a bit trickier.