Because AI is like software. Developing it is expensive, but the marginal cost of creating another copy is effectively zero. And then you can run it on relatively affordable consumer devices with plenty of GPU memory.
Search is also software. It did not move to consumer devices.
Search is more about data than software. And at that scale, the cost of creating another copy is nontrivial. LLMs are similar to video games in size, and the infrastructure to distribute blobs of that size to millions of consumer devices already exists.
Search is more about data, LLMs are somewhere between those two.