Because the mids and highs are enforced within the room lending a flat in-room response rather than a boosted response.

Now this of course depends where you sit in relation to your speakers. If you are sitting near-field, you are receiving mostly direct sound and hence an attenuated mid and high response. If you are sitting in the diffuse field, the response is flatter.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated