These airports do it to maximise their revenue from coach services (increasing the coach fares to the maximum and getting the best 'cut' they can from this revenue by competitive tendering and reducing on road competition). This revenue is then used to reduce the aircraft departure charges, which in turn reduces the headline airfares charged by the low-cost carriers, which in turn attracts lots of passengers to book those flights. Same with Car Park charges, drop off charges, retail outlet rentals etc etc., in fact anyway of raising revenue to reduce departure charges.
You don't really think that the low cost carriers can afford to offer £29 or whatever to Prague, without extra money being extracted from the passengers in some other way ?
They could, air tickets from Luton and Stansted could be a bit higher, as a result of that some passengers may rather go on a flight from another airport, so the air fares would get even higher because load factors would be less etc etc. Why would they want to do that?
Similarly, if the coach fares are a bit higher, passengers may also go on a flight from another airport. Living in NW London, I prefer Luton or Gatwick, and in the past I actually took Wizz Air from Luton even when there were cheaper Ryanair flights available from Stansted. Off-peak Oyster train fares from Gatwick is very cheap as long as the Express is avoided, compared to the airport coaches from Luton or Stansted, while Luton has very convenient Green Line 757 which stops right off the motorway at Brent Cross for the fastest possible connection. The extra fare and time for me to get to Stansted compared to Luton / Gatwick can easily be converted to a monetary value when deciding which flight to book, if there is a choice between airports.
The interesting question is why the Stansted Express train hasn't managed to dominate the market in the way the Gatwick Express did. Luton is slightly different as the distance from the airport to its station exists, even with the new DART link, and so a direct coach from the airport bus station offers some convenience advantages for passengers. Possibly it is down to the type of airline & so traveller, Luton & Stansted are both dominated by budget airlines so if the coach companies can keep their fares down then a comparison with the train becomes marked as often the train ticket to the airport is higher than the cost of the plane ticket (even on a more average rate and not one of the bargain couple of quid special fares these airlines can offer).
Stansted Express is slow, compared to other airport rail links, and it had run every only half an hour for a long time until very recently when the 15-minute headway was restored. Furthermore, Stansted Express is comparably inconvenient as well as it is at the end of a branch line, and it doesn't run through Central London in the way Thameslink does. In addition, the motorway options are much better from Luton and Stansted compared to Gatwick and Heathrow, in particular, it is a fast run down the M11 from Stansted to Stratford, or the M1 from Luton to Brent Cross, while there are no motorways from Gatwick to anywhere close to Central London.
Compared to other airports, Gatwick Express doesn't dominate the market but rail dominates the market because of the existence of cheap Southern and Thameslink options, and they are very frequent and carry you through Central London. Heathrow Express also doesn't dominate the market but rail also dominates the market because of the existence of cheap and frequent Piccadilly line and convenient Elizabeth line.
