Shoreham is an excellent airport.
It's not "dirt cheap" to land there but then going anywhere in a light aircraft is not cheap, and usually the landing fee is barely significant relative to the fuel burnt on any "useful" trip (i.e. one which one could not do in a car in less time).
If one spend a long while banging circuits, that is expensive, but why do that? Why not fly to France, or even further, and do something different?
The UK is not like the USA and it never will be, because here nearly everything is privatised, whereas the US aviation infrastructure is run on common taxes.
why do we need to fly from a massive lump of tarmac like Shoreham or Cranfield, when much smaller, cheaper, but entirely adequate places are available.
Because they are
not available, in much of the UK.
"Entirely adequate" for a Maule, maybe.