To expand on @knzhou 's comment some. The sun's rays are very parallel, but not exactly parallel. Most of the time we consider them parallel but really the disk of the sun is about 0.5 degrees. However when you are focusing the sun's light you are not imaging that disk, instead the rays that are within the numerical aperture of the lens (essentially all that within the diameter of the lens) are collected and focused to the spot.
On the other hand you light bulb is an extended source, meaning that if it is 5 cm diameter, and you are 50 cm away, the disk spans about 5.7 degrees. For most experiments you are probably trying to either image the spot on screen, or collimate the light (make a beam) using the light as a source.