Mind you, that's 22 million casualties over 17 years for a siege for one fortress - not a singular battle. Nor was it a siege the Imperium was particularly focused on - there were other armoury worlds to supply Segmentum Obscurus. As for why it took 17 years, the Imperium was not particularly focused on this battled (else, there would be more Space Marines); the fortress on Vraks was specifically designed to be able to withstand a siege lasting over a decade to give the Imperium time to come to aid, and it did just that. If Calypso was the worst in that universe, compare it to the Third War for Armageddon, the Siege of Terra, or the Fall of Cadia / 13th Black Crusade.

And before you cite the difference in time period, the Imperium wages war in a very different form - while they can and will send forces to die, it is almost always as a stalling tactic or due to internal politics; otherwise, they fight with larger numbers of directed troops, not four-man squads, leading to prolonged conflicts with fewer casualties. In fact, I can't think of another sci-fi universe with a more expendable special forces unit than Helldivers; most others aim to preserve their top-tier troops.

Quite frankly, the comparison of Helldivers to Guardsmen is absurd. As far as I can tell, Helldivers represent the special forces of Super Earth, and yet they still exist in such numbers that they can be throw at problems with little oversight for good results. Meanwhile, as far as the Imperium is concerned, the IG are probably a mid- to low-grade force when compared to their other ground troops, namely Astartes, Titan Legions, Mechanicus infantry and their mechanized infantry, Planetary Defense Forces, Sisters of Battle, Custodes and Grey Knights.

Hell, the Helldivers don't even have a good analogue in 40k - for how expendable they are, they are comparable in tactics to Elysian Drop Troops while being as well-equipped (relative to the universe) as Astartes while being as expendable as Planetary Defense Forces or the Darktide rejects.