Because they want it to be, it removes blame from them and makes them the victim. Oh noes, woe is me, I'm only a horrendous fat smelly lard arse because I can't afford to eat better. Bollecks!
When I cook, I do it properly. Make shed loads, feed whoever is about and freeze a load. Now when I make a chilli, I'd say it costs as much per portion as a frozen ready meal. There are a few differences though, mine is made with all fresh ingredients, I use steak as well as steak mince, I usually use 3 different types of chilli pepper, I have several other ingredients that cost a bit but really help the flavour. Despite this blatant disregard for cost, it still comes in at a similar price to a Tesco one. Plus my idea of a portion is double theirs!
I don't do it very often, as said I'm a bit of a scrooge with my food pennies, but when I do I have a meal I know I'll love and all it takes over the cheap ass microwave one is a few hours cooking. If I keep 5 portions, all I have to do is microwave them as needed. If I made meals to the standard of a Tesco ready meal it'd cost sod all.