Because "pretty" as a modifier generally means that something is relatively so, but not completely.
Like if you draw a line freehand and it comes out "pretty straight", it's relatively straight but not completely straight.
If you buy something that is "pretty cheap", it's relatively cheap, but still a bit on the expensive side.
Similarly, if something is "pretty good", it's relatively good but not perfectly good.
Because "pretty" as a modifier generally means that something is relatively so, but not completely.
Like if you draw a line freehand and it comes out "pretty straight", it's relatively straight but not completely straight.
If you buy something that is "pretty cheap", it's relatively cheap, but still a bit on the expensive side.
Similarly, if something is "pretty good", it's relatively good but not perfectly good.