Because I think on SW the main drive is to move away from overly processed foods and towards a healthier more sustainable way of eating. Ready meals tend to be full of flours, straches, thickeners, higher in salt, sugar etc.
And to be honest most ready meals are passable, and not filling. On WW I found it was like "Oh only 4.5 points for the brocolli and mushroom bake", but I would eat 2 to actually feel full. Not knocking WW btw, just personal experience. I found I would get more "bang for my buck" when I would make my own meals.