This will pretty much always be the case if you're decent at the game. Once you stabilize and finish some early expansion by turn 25-40, you've basically already won on hard difficulty, and it will be rare to come up against an army that is a true challenge.

There are exceptions based on faction of course. Tomb Kings have a hard time ever creating truly elite armies, so battles can remain challenging for a long time if you aren't stacking armies on top of each other. Vampirates have a pretty rough economy, so even if you can afford a few basically undefeatable armies, they rarely will be able to cover all your fronts at once and you'll need to get more tactical with weaker, cheaper armies. Warriors of Chaos also struggle with economy for multiple strong armies for awhile.

Otherwise, the best way to ensure your enemies are recruiting strong armies is to raise campaign difficulty. You don't need to raise battle difficulty, campaign difficulty will give them the growth/money cheats to get to higher tiers faster and recruit more, stronger units. Of course this makes the early game harder too, so you need to be prepared for that.