“Secure” doesn’t just mean “defend” or “safeguard”. “Secure” also means “capture”, so it implies you need to go out and get it, like with invasions or something maybe, idk. It’s a lot less passive.

“Existence” means both “survival” and “prosperity”. It implies that both their existence is under threat, while creating the idea that they might be able to make it better. Maybe they could, idk, make america great again, trying to return to some mythical prelapsarian era by beating back the eternal threat.

“A future” does this too, implying they may not have a future at all.

And talking about the “children” brings up the concept of making new people, evoking the idea that non-white races are out-breeding them. It also includes the common “think of the children” idea.

And finally the fact that it’s short and boring comes from the fact that nazis are uninspiring and uninspired, and they need a simple, to the point slogan to repeat without thinking about it too hard. They certainly know how to make a message concise, even at the cost of saying almost nothing of meaning when you think too hard about it.

All of this is classic fascism.