Because it's usually not said about other nations committing crimes against international law. Iran is allowed to exist. Russia is allowed to exist. North Korea is allowed to exist. Heck, Nazi Germany was allowed to exist.
The right to exist as a nation is such a fundamental idea to international law that I have to wonder how you would argue otherwise without stepping into genocidal territory.
And it is a bad look, to be completely honest, that of all nations is just so happens to be Israel to be the one country where people have continuously called for their destruction.
Israels current actions must be condemned, no debate there. And the German government should be louder in their criticism (although I can see why they of all governments have the hardest time doing so). But Israels right to exist has nothing, zero, to do with that.