Grace wound up in a "Sports" shirt because it seemed like the right thing to do after putting Tedd in that sweater.

When it came time to design the ruins of Washington DC for Fallout 3, Bethesda asked experts what would actually be left of the capital after getting nuked (these experts presumably had high Intelligence SPECIAL and lab coats with +10 Science). The answer was a somewhat unhelpful "not much", so Bethesda had to sort of ignore reality a bit to make the ruins worthy of exploration.

Not that realism is a primary concern for the Fallout series. Its SCIENCE! takes inspiration from 1950s science fiction, which means giant ants, giant scorpions, mutants, bulky robots, etc. The culture also seems to be frozen in some variant of the 1950s in spite of pre-apocalypse civilization lasting all the way to 2077.

Giant ants, incidentally, are physically impossible on Earth. Thanks to how objects scale, they would collapse under the weight of their own exoskeletons, and they aren't designed to breath in a way that would be necessary at giant sizes.

So no giant ants for us! But maybe for Grace and friends at some point because, y'know, magic and junk.