Because instead of spending minutes to hours collecting, sorting, filtering, parsing, and synthesizing Google results, chatgpt provides a near instantaneous answer that is often correct. It's then trivial to validate via Google.

Google is pretty good at short-answer questions (the occasional summary at the top can do a good job, and "people also ask" section can be pretty good) but if you ask something like "What are all the tax implications of moving to France?", those "smart" sections tend to be useless, and then you get sent on a wild goose chase of going to websites of varying quality.

On the other hand GPT will give you a really good summary of all the issues, personalized to your financial situation, and you can dig into details with followup questions. Then if you want to verify a particular tidbit you can go to Google.