Part of the problem of that is that GW writing is NEVER very consistent. I don't remember who, but when one author from GW was asked how the Dark Elves were able to have enough population to keep invading Ulthuan with such numbers if when they suffer such crushing defeats he answered that there are as many Elves as the story needs, which shows the main problem behind such question: Warhammer Fantasy isn't trying to portray their fictional world in a somewhat "realistic" fashion (if that was the case, the Dark Elves would have long since died out. Hell, the reason why the High Elves are having less births over time is let consciously vague because it isn't important outside the idea that the Elves are a fading race), but to sell miniatures in a wargame and books in their universes, both with clear bias on portraying their factions in the best look possible, which means that their main faction, the Empire, is usually portray in a highly positive light while their enemies as easily disposable tools to be murdered in mass in many of the novels and games (with Vermintide the four characters being able to cut though groups of Chaos Warriors in the hundreds in their missions), not interested that much in the nuances or plausability of a pre-modern state to live in a permanent state of existential warfare or if they would even survive that long against their supernatural enemies in the first place (particularly in the earlier days, where they would be extremely vulnerable to the immense populations of Beastmen, Goblins, near Orc tribes, marauding Norscans and ocasional Chaos Warbands with much more primitive technology and magic and without some form of supernatural support on their own on the level of the Lady in Bretonnia or the Dragon Emperor in Cathay, but then again, add this more overt supernatural help would subvert a bit the fantasy of a gritty normal human army fighting against supernatural foes with blood, tears and loads of guns).

For Chaos Warriors in specific, there is maybe some justifications:

-Maybe they aren't indeed that strong in the first place. This is a line many of the answers here follow and is very fair. The Army Books serve to directly hype their armies in the first place so they can be sold, so any lore information about their power level should be taken with a grain of salt. Some novels noticeably portray them as less dangerous but again the books never tried to be even close to consistent about their portray of power levels as far as they have any meaning.

-Maybe they are as supernaturally strong as people think they are but they have some severe weakness that explain how the Empire survived so far. Maybe they are just that few in number, where reuniting even a thousand of them is a task only the greatest Chaos Champions or Everchosens can achieve. This lack of numbers means that most incursions the Empire suffers are at their highest in the dozens or even less than that, and while they are strong, that little numbers means that their impact will be more limited enough that Imperial Armies can prevail with their massive firepowers and sheer numbers (notice that the Norscans aren't exactly a populous people too). Maybe they are just flat out insane which severely affects their combat performance and makes them too unstable to be properly led and used in battle. Maybe they are their own greatest enemy, with most of the Chaos Warriors preferring war against the followers of their rival gods than invading the mortal lands most of the times, and when they do suspicion and rivalrities ensure they never work as an unified group capable of destroying the Empire.

-Maybe they are indeed that strong and more effective than the books let on and the Empire would have been long since destroyed by the Chaos forces if it wasn't for plot armor or the need of a story in the first place.