In any worldwide disaster on earth, there are going to be a lot of people competing to survive. Building large fragile structures to, e.g., keep the now poisonous atmosphere out because people outside of those structures will have a tendency to destroy them. On mars, you don't have that, you have a relatively stable society where everyone is in one boat together.
If your on mars you also know exactly what you're going to face. The temperature, lack of significant atmosphere, and dust storms. That's pretty much it. You can prepare. On earth you don't know what's going to happen next. Significant die off of oxygen generating plants creating an unbreathable atmosphere? Lack of water? Too much water (flooding)? Hurricane? Nuclear war (or super volcano/asteroid, but those are less likely) throwing up dust and blocking out sun? Unmaintained dam breaking upstream? Unmaintained pipeline poisoning water supply? Earthquake? You can prepare for any one event, even in a post apocalyptic world, better on earth than on mars. But you can't necessarily prepare for them all as well. And even if you could, you won't be motivated to because you can cut corners.
Is Mars somehow immune to volcanos and asteroids?
Man some people really want to get off this planet! Earth has given birth and sustained life for so long, and now earth is "unpredictable"!
No comments about your other "arguments".
Asteroids, no, but it is likely more immune to long term dust clouds in the atmosphere as a result (since the atmosphere is so thin). On the other hand dust storms are similar.
Seems like we are not really sure about that...
https://www.newscientist.com/article/dn11962-lab-study-indic...
I am not sure. Can you say how we have been looking for seismic/volcanic activity on mars.
It's implausible that one will hit us, but if you're looking for a scientific and logical motivation, it's a possibility.
Also, colonizing mars is a necessary step toward avoiding what happens when our star reaches the end of its life. May as well start while we have the opportunity.
No, it is not plausible.
Most bodies in the solar system easily fit into a hundred astronomical unit radius sphere. The nearest star is four light-years away, and a light year is eighty-six thousand astronomical units. The ratio between those distances is 2530[1]. It'd be like hitting a meter size target from more than two and a half kilometer. Pretty good shot, and we're talking about Proxima centaury (which, last time I checked, won't go supernova anytime soon) and the distance between say Sedna and the Sun. If we were talking about an accuracy more of an astronomical unit, more like the distance between Earth and mars, then we're talking about a centimeter sized target.
Thus a gamma-ray burst would have to be insanely narrow to hit Earth and not mars. Not to mention how ridiculously unlucky we would have to be then in order for it to be precisely aimed at us.
1. https://www.wolframalpha.com/input/?i=four+light-year+divide...
Some people don't want all of our eggs in one basket. The chances a planet gets destroyed is a bit higher than a solar system or multiple planets.
Also one counter-argument to this is "But then why not the Moon?" I don't have as strong of a counterargument for that other than without the Earth's gravitational pull keeping the Moon in orbit, maybe it will get knocked into the sun or something when Earth is destroyed. Also I'm pretty sure international treaties and such prevent colonization of the moon.