system
Given the following post, respond as if you're commenting on Reddit:
human
[40K] Why are Space Marines portrayed as battlefield demigods when they seem more akin to simple tough walking tanks?

I get that they're post human and can survive what would kill most, and are smarter and more mobile than tanks, but they are portrayed as being able to take a planet like 21st century earth with a single chapter relatively easily. This seems ludicrous when one realizes they're just big bipedal armored durable people.
gpt
I think you're oversimplifying things... 

Since the Horus Heresy, Space Marines are rarely used in pitched battle. They lack the numbers. Instead, the operate as rapid insertion specialists, dropping where the enemy is weakest and doing damage before withdrawing.

Within 24 hours, a full strength Chapter could kill every political leader on earth, along with the majority of General rank military staff, disrupt supply lines and infrastructure and generally incite havoc amongst the population.

Even if committed to field engagements, Astartes Power Armour is more durable than any contemporary tank. They can withstand direct hits from Krak Missiles (though not frequently) which are superior than the vast majority of modern AP explosives. 

Physically, and Astartes is superior in every way to a human, able to survive with absolutely unfathomable wounds (there is an old story of a Marine continuing to fight after his rib cage had been blown open, his organs in full view). They are fast enough to intercept low-velocity rounds, strong enough to throw a car, and capable of moving and fighting at speeds in excess of 60kph. All of these capabilities are then heightened by their armour. 

In short, it is the combination of their armour (which surpasses a 'simple tank') operational mentality and physical superiority which makes them demigods. Yes, there are weapons which can take them out even today. Problem is, you have to have those weapons on hand, everywhere, at all times. Because the Strike Teams are just going to avoid the positions that have them otherwise. 

***edit
Another major advantage... Astartes do not follow the rules of war. They will target civilians to draw out military positions, they will kill the weak and sick, and they will deliberately massacre children if it means military gain. They also make wide use of chemical weapons and incendiary armaments which are highly ilegal.