They're actually designed to perform best when not clipping.
-18 dBFS RMS is just a loose approximation of what 0VU (coming from the outside world) will fall on the DAWs meters. Think about it, 0 VU can't be 0 dBFS or you'd instantly clip if your analog gear ran > 0VU, because it can, so the reference number is "pushed down" on the digital scale to give wiggle room, that's all there is to it.