https://en.wikipedia.org/wiki/Big_dumb_booster
There's also many advantages to being able to lift something large/heavy in one go, rather than smaller payloads that need to be unfolded (like JWST) or assembled in space, which can drastically increase the development costs.
Operations cost. They are sublinear on payload/size. At least this is what Space X/Musk seem to go for.