Because the beauty of the BBC is that they don't have to be so obsessed with ratings. In theory that should allow them the option to be more adventurous and creative than a commercial broadcaster who would understandably keep churning out episodes of Holby City and Casualty as nice little profit centres.
This is a much more reasonable objection at least in regards to Holby City (and Waterloo Road). The money saved from reducing the episode order of those shows isn't going to be enough to invest in new pre-watershed drama although maybe with a little extra investment they might get some pre-watershed comedy. And certainly they'd be able to try some entertainment formats during the week as well.
Casualty on the other hand desperately needs an episode reduction. It airs far too many episodes over the course of the year and can make scheduling Saturday nights difficult. Equally there's quite a bit that the BBC could do with that later Saturday night slot that wouldn't cost a huge amount of money if they didn't have to work around Casualty too many weeks of the year.
Personally I would move Casualty back to Fridays, moving the EastEnders episode to Thursday at 8.30pm to make way for it. The 8pm EE filler could then largely be comedy (well, it would need to be to counter two episodes of EastEnders on the same night!)
Saturday nights can more than cope without it - for a start the BBC can stop recording results shows and instead play them out live after 9pm on Saturdays, while on weeks without Strictly or similar the BBC generally only need to find 30 minutes of additional programming to replace 50 minutes of Casualty by simply making a few tweaks here and there - or even less if they started MOTD at 10pm.