Why do George and Lydia suppose that they cannot change the nursery?

George thinks that the room is broken and Lydia thinks it is in a rut because the children have been thinking about Africa for too long. 

The Hadleys invested in a HappyLife home, which is a fully automated house that does everything for you.  One feature of this home is a special nursery for the children.  The nursery responds to the children’s thoughts and projects scenes on the walls, making the room seem to transport to different places. 

The walls were blank and two dimensional. Now, as George and Lydia Hadley stood in the center of the room, the walls began to purr and recede into crystalline distance, it seemed, and presently an African veldt appeared, in three dimensions, on all sides, in color reproduced to the final pebble and bit of straw. 

George and Lydia are concerned because the room seems to be taking over their children’s lives.  The kids do not want to leave the nursery, and refuse to turn it off.  The Hadleys are worried that their children are having dark thoughts, and those thoughts are turning up in the nursery. 

"The fool room's out of order," he said. "It won't respond."

"Or--"

"Or what?"

"Or it can't respond," said Lydia, "because the children have thought about Africa and lions and killing so many days that the room's in a rut." 

They even hire a psychologist, who tells them to get rid of it.  The room was very expensive, costing “half as much again as the rest of the house.”  However, the Hadleys are losing their children.  They do not feel a connection with them.  Mrs. Hadley is frustrated because the house seems to be their mother and they are too dependent on the nursery’s entertainment.

The Hadleys should have intervened earlier.  They are too late.  The children lure them into the room and kill them.  The psychologist sees them watching the lions eat their parents.