For such people, it's a lot more important to signal familiarity with bourgeois/middle-class values than economic status per se. So it's one instance where one's outfit might actually convey useful info to outsiders. But even then, they don't have to expend huge amounts of money/effort; they just have to not screw it up.

If you look at photos from many decades or even, say, a century ago, back when middle-class values were truly common across the socio-economic spectrum, you'll see it was a lot more common for even low-income folks to wear suits or at least semi-casual wear. The point was not to waste resources or signal wealth; everyone knew that these people were quite poor. It was all about values.