We need sizes above XL for women, otherwise they are forced to choose men's sizing and the fit just doesn't look the same as the fit for women, so they either have to settle for men's fit, or they will abandon the shirt and find one that has their fit and size, which is more likely.
Right now merch only offers sizing up to XL for women, while men have 2XL and 3XL sizes, why not for larger ladies? Statistics show us that there are over 100 million plus sized women which is a huge market, but the apparel industry doesn't match the demand.
The impact of offering bigger sizes would be more sales, women are huge online shoppers so catering to them and their needs will make a big difference. (as well as other suggestions like offering pink and purple, v-necks and tanks etc)