Because simply put, rats are omnivorous. If you don't provide meat products to their diet, you're keeping essential nutrients from them. I do not recommend you feed them raw meat at all.
If you try to make them vegetarians, you will be giving them an incomplete diet. They do not have vegetarian digestive track, they are not ruminants with multi chambered stomachs and have the ability to break down plant matter easily. They have to have a second go at whatever plant matter they eat by eating it again.
Give them too much of one thing, or give them the option to pick and choose, you can end up with either fat rats (from them picking out what tastes good), or an underweight sickly rat, (because they lack certain vitamins and minerals).
My advice is to NOT talk to people that feed a RAW only diet, because they will give you what you want to hear. Talk to people that feed a home made diet, people that have worked with rats for years and years etc etc. It will give you a balanced out look into rat nutrition. Not just what you think is best.