Why do I have to keep signing in on google

Safari itself is not a Search Engine, but you can choose from a number of different Search Engines it uses.

To change the Search Engine used by Safari, refer to Choose a search engine in Search for websites in Safari on iPhone - Apple Support.

If you have a Google account, I can only surmise they want you to do that so that they can monitor your Internet activity, which is what they do. You would need to contact Google for a definitive answer. I don't use Google so I really can't help with their products and services.

Contact a third-party vendor - Apple Support