You can set your own stat weights, but you have to first generate some data that is roughly compatible with your character. This works the same way that it did in Legion.
The only difference is that in Legion we had the “imaginary” strategies that let you specify everything by hand. Nobody really used those though… or if they did, they didn’t fill in the value for any of the special stuff (like set bonuses, legendaries, trinkets)… which results in a pretty useless gear ranking.
You run into the same problem in BfA – unless you specify a value for azerite powers and trinkets, just stat weights aren’t very helpful. So we have a method for you to calculate the minimum set of data required to rank gear reasonably well, and then you can customize the stat weights as you see fit.