To avoid the lie automatically succeeding. As I can explain below.
Say an NPC gives the PCs some misinformation, but the PCs don't express any doubt or inquiry. And in this case our DM is not taking an acting approach to their presentation of the game-world, or perhaps they think they gave the right subvocal clues, but failed dismally. To me, the characters inhabit the world... so shouldn't they have some chance of noticing the deception? Just as the characters living in the game world have a chance to notice an NPC sneaking up on them, even though there is no one acting that out at their rl table.
There are many approaches that could work here. One is that a DM tells the players that they might be being lied to. Another is the DM decides to make a check for the NPC - CHA (Deception) against passive WIS (Insight). That comports well with other cases covered under RAW (such as where an NPC attempts to be stealthy).
Put simply, a DM can always rule something is uncertain, and are encouraged and endorsed under RAW to do so. The alternative is that in the case at hand, the NPC automatically succeeds.