It's not that simple. They have to cut a size range based around the median in the size run, it's to do with how the larger and smaller sizes scale on the available material relative to that. It wouldn't be cost or logistically efficient to make literally half the clothes one size, unless prices went right up to account for all the unsold sizes and wasted material that would come about as a result.
However, if 16-18 is by far the most common size, it will eventually become more important in how the other sizes are scaled (at least until the proportions no longer work and you need a size break) Since size numbers are based on archaic scaling systems rather than specific measurements (it's much easier to scale and pattern cut around a scaling system than an individualised system for each size), plus the fact that there's no standardisation and it's harder than ever to profile customers, you'll find that changes the sizing dimensions. That's what people call vanity sizing, but it's nothing to do with flattering plus size women, much as I know nobody wants to hear that.
You probably don't want standardised sizing anyway, or people with big boobs or long legs really will never find anything to fit. You probably do know which places tend to cater better for your shape; do you want to lose that?
Also, it's worth remembering that you need to keep a realistic idea of how perfectly a mass produced item is likely to fit any random person.
Edited to correct an error, sorry.