I'll tell you why.
First, they require no effort. Yeah, yeah, I hear you all saying "that's the point!" You want to throw something on that is cute and comfortable, because you are just running to the grocery store or to Target or to meet your girlfriend for coffee and you don't want to get totally dressed in an actual outfit. When I say they require no effort- I mean they are in no way an example of personal style. Everybody looks the same in them! If you tell me that your set has some phrase across the ass that makes it "unique" I am going to barf all over it. Stripes down the outseam aren't any better. They are also ugly.
Since I just barfed all over your lounge set, I feel that the only time and place appropriate for this outfit- is AT HOME- doing laundry. In my opinion velour lounge sets are just 1/2 a point above Old Navy pajama pants on the scale of casual-ness, and unless you are 15, you shouldn't wear Old Navy pajama pants out in public. I know, you love wearing them on the airplane, because you feel so chic and jet-setter-ish, but wear something else. Wear something other than almost-pajamas. Here are some cute AND comfy ideas. (you can find them all on shopbop.com) You're welcome.