Is someone able to explain with simple words and short sentences why a person should not have 100% shares?
Because investors have human emotions, and are subject to panic and making self-defeating moves to their portfolio.
Someone who owns 100% stocks is more likely to panic and sell the next time the market drops 50%, because their portfolio will also drop 50% and they'll think the world is coming to an end. Someone who owns 80% stocks and 20% bonds won't see their portfolio drop as steeply, and thus, will be less likely to defeat themselves.
You might think you wouldn't be stupid enough to panic like that. But until you've lived through it, you can't say for sure.
Some reports show that the average investor's return over the long term is only a small fraction of what the market returned. That's due to their poor choices. Maybe you'll make better choices than the average investor. But the odds are against it.
I get the feeling that people asking "why not 100% stocks?" are relatively new to investing, and thus, have never experienced anything but a continually rising stock market. Or, even if they were around for the crash in 2008, saw their portfolio cut from only $10,000 to $5,000, rather than $1m to $500k.
If you held multiple decades worth of living expenses invested in 100% stocks in 2007, and still held 100% stocks in 2009, then congratulations, you are more robotic than most, and staying 100% in stocks going forward will probably produce the best results for you.
But for many other people, being 100% in stocks today will not produce the most money in 30 years.
(about dividends: they aren't very relevant. They can be cut/eliminated, inducing probably even more panic than the share price drop alone. And in the US market these days, earnings tend to be retained more than they're paid out, so selling shares is going to be part of most withdrawal methods.)