Wages are decided by free market forces, except where we interfere in some way to say they aren't.
It's not necessarily wrong to interfere - that might mean setting minimum wages, or having a union to negotiate (though really that's the market) or laws against paying people differently based on race or sex, but it's important to understand the primary driver of wages is what workers are willing to work for. And there are cases where workers, even professional ones, are willing to have a lower wage for a more flexible job.(Or you could look at it the other way, the employer is paying a premium to workers who will be available for any shift or extended hours.)
No business can pay workers less than they are willing to work for, whether or not some egghead has decided that one type of job is "equivalent" to another. If it is easy to get warehouse workers, but not shop clerks, the question is if the work is equivalent, why is there this differential? The metrics used to determine that kind of thing are pretty crude (and I think this is the main reason people resist that kind of comparison - they are obviously so crude people see they are different kinds of work.)
PT vs FT does also can affect value to the employer in real ways, it' s not only a neutral value. Especially when there is a squeeze on the number of workers, where there is an element of expertise or continuity issues, or they are investing in training. And that should not be a surprise, because it also has a value to employees - lots of people will choose a certain amount of flexibility, or fewer hours, for a lower wage.
I'm not sure why people keep thinking you can calculate backwards from some absolute value of the product or service to determine wages. The cost of the final product is determined according to the costs to produce it, including wages. If the employer needs to pay a certain amount to workers (whatever he can get workers for, which should generally be baselined at a living wage), that will be accounted for in the final price. If the final price is too high and people won't pay it, the product isn't viable.
If you determine wages dependent on the final value of the product, that means a product that sells only for a tiny amount would only need to pay workers a tiny amount which would be clearly unfair.
The changing face of a lot of employment is also going to change this attempt to calculate female vs male dominated jobs in a lot of cases. There are a lot more male shop assistants now, and a lot more female warehouse workers, so it's much harder to argue that a wage differential is based on sex discrimination.
All of this is to say - wages are already determined mostly by the market. We put certain lines around that to prevent exploitation, but they are not based on some kind of artificial setting of a wage based on an intrinsic value of the work.