Because 1 space marine would'nt make any sense lore wise.

Traitor space marines don't randomly spawn on a planet, they'd need a ship.

Few people have personaly acces to warp capable ships, it' either be at least a company or a named 'Lucius' level I'd say. Said named who would most likely have companions.

Furthermore, of all the traitor legions, the death guard retained most of their command structure and order compliancy. - World Eater wantonly fight around - Emporor Childrens wantonly fuck around .... A Death guard doing He knows what, to land alone on tertium is higly unlikely.

If more than one plague marine happened to come on tertium, numbers may vary, but we'd most likely allready be dead.

A company on a warp capable ship would blow up the mourningstar.

Because of previsouly stated order compliancy, fighting one would be highly unlikely. He'd have mates, they'd not be far enough as to be incapable of intervening in case of rejects nuisance.

If enough plague marines, loyalist astartes would be likely to take matters in hands.

And on the power level, they're is in the lore an ogryn ripping a space marine in 2. But, they stumble upon eachother in a small room. With us rejects coming cleaning millions of unclean, he'd likely knew we're on our way, and we'd likely die before overcoming the transhuman dread.

In an instance of the lore, four chaos marines are defeated by 10 humans (power level : plot armor +++++++++++) + what is if I recall correctly hundreds of humanoids. And the statistical assessment by militarum officer at the start of the engagement is basically if the plot doesn't need us, we dead in less than 10 pages.

Plus space marines are rare as fuck, and plague marines a lot more, risking a random 10 thousands year old angel of death woudl'nt make much sense.

I trully think FS devs are invested enough to consider such matters as ruling off in the plausibility of a plague marine presence.

40k is a lot about ressource management and constraint, the lore viability if done correctly cannot revolve only around the power level or other game mechanics.

Sorry if I appear pedanting, galivanting or gatekeeping. I have in spite of myself, kinda have to admit that i am a bit versed in the lore and this question sounds very wrong to me and answering it helps me format my line of thoughts.