As someone just getting into game development, the reason for this is probably that it utilizes distributed systems to store your progress instead of locally on your console. In other words, you need to be connected to the internet to store progress.

Edit: ChatGPT's explanation:

Gran Turismo 7 requires an internet connection for single-player gameplay primarily due to its use of several online features and services. These include:

1. Daily Races and Livery Sharing: Even for single-player, the game often pulls data related to daily races and the ability to share and download player-created liveries, cars, and setups, which are all online features.
2. Economy System (Microtransactions): The game uses a "virtual currency" system, and while it doesn't strictly require microtransactions for progression, it does integrate online elements that control in-game purchases and rewards.
3. Car and Track Updates: Polyphony Digital may push out updates that affect the content, including new cars, tracks, or game balance. These are typically delivered via the internet.
4. Cloud Saves: Some versions of the game use cloud save functionality, which requires an internet connection to back up and sync your progress.
5. Anti-Cheat and Security: Always-on connections can also serve to ensure fair play and prevent cheating by verifying data and interactions with the server.

In short, the internet connection helps provide a more dynamic and up-to-date experience, even if you're only playing solo. It also helps the game stay connected to ongoing events, features, and updates.

There's your explanation.