One slit is not a point — every part of its opening re-radiates, and those Huygens wavelets interfere. The result is a bright central band flanked by weaker side maxima. Counter-intuitively, the narrower the slit, the wider the light spreads.
Try this: drag the slit width a down to its minimum — the central band balloons outward. A narrow gap forces the wavelets to spread over a wide angle; a wide gap keeps the beam tight and collimated.