Government paid healthcare is still a big expense that's covered.
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.