Why have database security?

Now, I'm pretty new to the world of databases, but it seems to me that the most secure, risk-free option is to simply not store sensitive data.

On the most basic level, this might just be not using it, or perhaps you have users use third party systems like Google or PayPal to shirk off responsibility, but the point is: if people suck at setting passwords (which they do) and processing power is increasing exponentially (which it is), why should a small business or someone running a personal website go to all the effort? Leave that to the big fish.

Can anybody please explain to me why nobody seems to have taken the most secure route of needing no security at all?

I'm trying to write up a small blog using MySQL and PHP, namely for practise. I want to have users who can write comments, subscribe to a mailing list if so desired, and it'd be cool to store their IP address/the country they are from too; however the main purpose is to keep track of certain users across different comments/posts (especially so that they can edit and delete comments). I keep going around in circles about security, so can anybody please help me think of how I could implement a database on the premise that it should have as little security/sensitive data as possible? Just as a thought exercise, to re-evaluate why we do things the way we do. Otherwise, could you please suggest why this is a horrible idea.

Do we even need passwords? Could a user log in via email or their IP address alone (supposing that users did not share their emails or that each device only had one IP address, rather than changing networks)? What would be the rammifications of such a scheme?