Because there's no need.
El Guerrouj had Lagat and Morcelli as opponents, who were 3:26-3:27 runners.
They pushed each other limits.
Today there's no one to push Jakob's limits beyond a low 3:28 maybe high 3:27 if Tim really gets back in peak shape. He just doesn't need it to win anything he wants.
The only way for him to run under 3:27 would be to do a Cheptegei style WR event, but I doubt he's really interested to do that.
I think the mental part is huge. Jakob is in a comfortable place where he is right now in the 1500/mile, and he subconsciously knows it.