A couple of reasons. Good condition manual boxes are hard to find, new are more expensive than power. In the case of my son's Barracuda it was not at all about effort, but was about turns lock-lock while he was autocrossing and 16:1 manual boxes are even harder to find (yes, parts are now available and expensive).
In the case of my Barracuda, I want my wife to be able to drive it and I have given up on finding a good manual box anyway. It had power steering in it when I bought it. She hasn't driven one of the big block cars yet. Her daily driver from when she was 21 is sitting next to the Barracuda. It did have factory power steering and little else beyond being a 340 GTS convertible.
My son's Dart is getting power this time around for the same reasons that my Barracuda has it. He wants his wife to drive it, once at least. And, his right shoulder is worn out. He can still shift, though. Now, I need to get a seal kit for his steering box...
None of the cars have power brakes, however.
