Where I grew up, Bristol Airport was a 20 mile drive away (public transport connections being quite poor too) whereas Bristol Parkway was only three miles - 15 minutes on a bike. Where my parents now live on the other side of Bristol, Bristol Airport is now a 7 mile drive, but the nearest railway station (that gets a frequent regional service and a roughly two-hourly direct London service) is but a four minute walk.
Ok, so take Manchester. If you live in Altrincham you are five miles from Manchester Airport, or nine miles from Stockport with an hourly train if you don't want to drive. OK, so the airport is possibly more convenient here if you don't want to drive. Mind the drop-off charges though. If you live in Bury the airport is 22 miles away. Piccadilly is 12 miles away with the option of a tram that only takes 9 minutes longer if you want to avoid driving in the city centre. If heading north, Bolton is six miles away and the trains to/from Scotland now call.
In Liverpool, the airport is 15 miles from Kirkby. Lime Street however is only 8 miles and South Parkway 13 miles.if heading north, St Helens Central is 9 miles away.
Cardiff airport meanwhile is further from the nearest Cardiff suburbs than those suburbs are to Central, let alone any of the South Wales Metro stations.
You see the issue? Most cities do only have one airport, which means that for half of the suburbanites it's on the wrong side of the city anyway. Whereas most cities in the UK (among those big enough to have an airport) will often have multiple suburban stations covering several angles, including ones which are directly served by intercity services.
Birmingham? The airport has an intercity railway station so no matter where you live it can only equal, not exceed the driveability from a suburban home. If you live anywhere that New Street, Coventry, Wolves or any of the Trent Valley stations are close to then train wins every time for domestic journeys.
And no one lives right next door to an airport terminal. Whereas most railway stations in cities offer direct pedestrian access to housing, shops and places of employment.