Actually, they are. They've put out a few old-school episodes with alternate video tracks of CGI-ed effects. I am not sure if it's just a 'testing-the-waters' phase, or whether they're tight on budget, but it hasn't happened to every release. The best part about it is the option of having either versions to watch.

I don't have a problem with watching old Bakers. Mainly due to Tom Baker's charisma and delivery in the part, and also due to the witty dialogue in most all of his scripts. And, then there's the whole Douglas Adams era. Watch City of Death again sometime. Sure, the guy's wearing a rubber mask, and all. But the Adams/Baker combo will remind you why the 4th Doctor is considered to be the iconic incarnation...