'If' is the problem. As it is, selling clothing means following trends and offering fast-fashion for a large part of the market. There is a sizeable group of people who are completely fine with getting the latest trend cheaply at Primark. These people might like real pockets (and post memes about it on social media), but you won't activate them to actually make that a requirement. The people who do find brands which simply do offer it (the economic model works so far), and those brands know that this is a feature people like, and they can afford to invest in the extra costs this brings (sewing, design) and the raised cost of their end product. But a large part of the market simply won't bear those costs.

Pockets on dresses is not much different from sustainable clothing (both in terms of production and in terms of durability) and fair pay for people who work in the garment industry: most people do not care enough to change their behaviour (or can't afford to).