Because listening through a speaker at distance is different to catching the sound right next to your ear. It is the equivalent of standing a few meters off axis from a 4x12, or having your ear right next to the speaker. Of course it will be bright and nasty sounding that close. If you doubt the effect, try (at a reasonable level to listen to your favourite distorted guitar sound with your ear next to the speaker.
And why going through the headphone for stereo, using the space effect and pure cabinet lessons that effect.