>>3067779

>Short answer:

To simplify implementation, draw more polygons, and reduce costs.

>Medium answer:

PSX graphics hardware interpolates polygon textures in 2D rather than in 3D in a sense. Textures are interpolated in "screen coordinates" rather than in "perspective correct coordinates". Without going into the math, a picture is worth a 1000 words.

On the left side of the figure is perspective correct texturing. It requires a computationally expensive "perspective divide", which has major repercussions on the design of a graphics pipeline. It's much easier to implement the right side, ignore perspective effects, and just interpolate in 2D. Here I have broken drawing a triangle into applying the texture to 2d triangles, scaling them, shearing them, and finally drawing them. Notice how the visual artifact of wobbly lines arises. This is precisely the thing OP is asking about.

The perspective divide makes things difficult because it can't be implemented as a matrix equation, like everything else. It breaks up your pipeline into before and after divide parts, which necessitates several matrix multiply units. It's not trivial to implement, but clearly is desirable to the point where we are willing to complicate matters to get the effect.

>Long answer:

look in a computer graphics book on how a graphics pipeline is implemented. Perspective divide and homogeneous coordinates are the most complicated part of the whole process.