It doesn't "take" centuries, it's just been going on for centuries. You can probably develop a very unique cultivar in a single lifetime. This is quite common in the horticultural industry and is especially feasible with weedy species like Brassicas

And the stability of the traits is mostly due to careful management. Most of these vegetables will very easily hybridize

In the Andes there are still traditional farmers that maintain over 300 varieties of potatoes. Each one has a name and a history. Some are only ornamental, some are only eaten in soups, some are medicinal, some are a bright purple, some are extremely long, some look like giant pinecones. Just look at the incredible images in this article

https://www.theguardian.com/environment/2019/nov/29/how-peru...