It's all down to cost.
There's all sorts of paperwork and vehicle acceptance issues involved in converting a Class 442 EMU into a Mark 3 locomotive hauled vehicle by splitting the sets, removing the defunct power cars, converting the driving cars to be compatible with a propelling locomotive, gauge clearing the stock (for it's not a standard Mark 3 - different bogies, doors and overall profile) and so on.
You only have 96 vehicles available by the time the central power car is removed, which it sort of needs to be as it's so heavy and as such, a really costly vehicle in terms of track access charges. It could be reworked into a regular vehicle, but that would be enormously complex in terms of either vehicle acceptance or engineering works.
You then have a proposal which everybody is happy with, and an engineering team begin work, already having spent the equivalent of the cost of a new bodyshell on the designs and paperwork needed to turn a Class 442 into a loco hauled vehicle.
The engineering team set to work on what is a near 30 year old vehicle last significantly looked at almost 10 years ago and find significant rust problems, wiring issues and other unexpected works which weren't budgeted for. The metal after inspection from a metallurgist shows issues of weakness and some structural components are unlikely to see another 5 years of service without being replaced, suddenly you've spent 75% of the cost of a new LHCS vehicle with absolutely nothing but a big pile of rust on the floor of a depot to show for it.
It's easy to forget that British Rail Engineering was, at times, as bad as British Leyland at screwing things together, they used the same or similar raw materials as BL and Austin Rover - just think, the Class 442 is probably made out of the same steel as your long since rusted away Maestro, it was painted in much the same way too when it was new, so underneath and behind panels, BREL's 1980s products can be a bit ropey.
Why would you bother trying to save 96 to 120 vehicles when they're really worn out and pretty much ready for the scrapyard now ?