It's not necessarily the fault of the TOCs. The DfT play a big part in where rolling stock ends up, though that is behind the scenes in franchise negotiations, so it's the TOCs who take the flak.
SWT had to increase capacity as part if its 2007 franchise. On the Portsmouth route the only way to do this was to replace 10-car 444s by 12-car 450s on some of the peak trains. In addition, with the commitment to go to two trains per hour to Weyouth, 24 442 units wasn't quite enough to maintain this service all day. They could have had a mix of Desiro/442 trains, but in any disruption it'd be harder to recover the service. In the old franchise the 442s essentially opertaed the Poole/Weymouth turns as self-contained diagrams.
It also turned out cheaper to lease the 458 units from Porterbrook than 442s from Angel. Thus as part of the requirement to increase capacity, a stock reshuffle saw the 458s kept (remember virtually the whole fleet had been stored at one point) and the 442s leave.
SWT supposedly had plans for a major interior refurbishment of the units, but the way the new franchise was let meant the units left - despite over a third receiving the revised long distance livery as part of their overhauls at Ilford.
When Southern took on Gatwick Express, the decision that led to that was to increase capcity on the Brighton mainline into/out of London in the peaks. There were no extra paths to this, hence the extension of certain GatEx services to the South Coast. Now the 8-car 460s weren't suitable to do this, and Southern had no spare stock to operate these trains. The 442s were off lease, and thus Southern ended up having to take these units. The move now to take the remaining 7, and put the 460s off lease, will allow Southern to remove a fleet type, i.e. the 460s. Operating all 24 442s will allow it to standardise the GatEx operation. I agree the 442s aren't ideal for an airport service, but that dedicated service is being subsumed into the perceived greater need to serve the South Coast (capacity wise). Plus Gatwick is not the airport it was, with the Low Cost carriers increasingly dominating, and a lot of the longhaul flights now at Heathrow (particularly trans Atlantic flights, where you used to have a lot of early morning arrivals). The demand for a premium airport service has dropped off, and many people are happy to travel on Southern or FCC rather than on GatEx, where it costs more.