Because they are not available, in much of the UK.
"Entirely adequate" for a Maule, maybe.
Most light aeroplanes can cope, most of the time, with a 800m runway, with a bit of clearway at each end, and a small clubhouse. The area around that runway can be productive farmland - it doesn't need to be "sterile" and regularly mowed grass, crosswind runways are seldom used (but certainly nice to have, but that runway can also run as a strip through farmland).
Lights and an NDB off the end of the runway are certainly nice to have, as are hangars but could be done fairly cheaply. Or (Goodwood, Thruxton...) you can shove the runway in the middle of a racetrack. We all drive to the airfield anyhow, so it doesn't need to be on expensive real estate close to a town. A/G is good enough for most purposes, and GPS approaches if we get there in the UK would negate any other instrument approaches for other than training purposes.
I like Shoreham, don't get me wrong - I have shares at Cranfield and Booker, and in particular like the option of an instrument approach at EGTC, or lights at both - but most GA flying, most of the time, can equally be done for much less money from a Popham or a Sandown.
G