Why should a retail investor never buy derivatives? spreads?

Retail investors do not have access to systems that calculate risk, margins, pnl, etc... and generally also don't have the necessary knowledge and market data to price such instruments correctly.

Most ppl are better off KISSing and lowering risk by selling equity for fixed income.

You almost always lose a lot of money if you're seeking safety. Protection from downside risk on your S&P500 investments may cost 20-30% of your investment at which point you're better off just selling the investment and hoping it doesn't go up by that much.

It’s scaremongering, you can learn all this stuff.

However! If you don’t want to learn and want to get rich quick instead, stay away.

Not the parent but I'm guessing: a) it's expensive and b) you can shoot your feet off.

It's all about getting a call from the dreaded Margin.