You are conflating lo-fi art with cheap and/or bad lo-fi art. "Cheap lo-fi art that looks good" is not a thing, because the talent that can make something lo-fi also look good and, more importantly, provide good art direction for an extensive project is not cheap.
We are not talking a screen full of pixels or two. These games would require thousands upon thousands of art tiles, that all aim for a distinguishable, consistent and unique in overall style.
As you said, it's hard. That makes it expensive, in either time or money, or both.