Come back to what exactly? If you haven't contributed to the social safety net, you're not going to get much out of it, and by the time you're 60 chances are a lot of things could change, for better or worse.
Which makes perfect sense; if it happens at scale, how would that safety net survive?
Right now, the few who do this are free riders on a system everyone staying behind is supporting. It's not a problem if a thousand people leave Germany to go make money outside the German tax regime and then return in their old age, their system can absorb it.
What would happen if a fifth of the population did that? It might be harder to sustain that level of social services with that loss of tax revenue, i.e. payment for those social services.
Being shackled to the mediocre majority is never fun, some feel the shackles more than others.
They don't like the safety nets.
I guess they'll have contributed to their personal wealth instead and will just come for the public services / safety offered.
Safety? I'm quite sure Singapore is a much safer country than Germany. The US, not so much...
But if you've already moved to Singapore and you want to live in a safe country when you're older, it would make no sense to leave Singapore, unless maybe you're moving to Japan. Western Europe is of course generally safer than America (very little gun violence, after all), but Singapore is one of the safest places on the planet. Just don't walk on the grass.