Is it patriarchy that women's pants do not have pockets? I say no, and here's why.
For one, the premise is just incorrect. Plenty of women's pants do have pockets, and rather large ones, too. Even dresses and skirts can come with roomy pockets. Google "women dress pockets" and you'll find sections on websites dedicated to women's dresses with pockets.
So maybe we need to change the premise: is it patriarchy that NOT ALL women's pants have pockets?
I continue to say no.
First, this assumes pockets are always desirable. Women who like pockets assume all other women and people of all genders naturally like pockets, and want large ones.
There are reasons to not want pockets, though:
maybe we might say that preferring slim clothes for feminine beauty standards such that pockets are not an option is a form of patriarchy. that's something we could discuss.
at the same time, it's also part of trend cycles. modern fashion cycles from slim to large and back. mom jeans were in style a few years back and they had huge pockets. Now it's all about flares. you can find options with or without pockets. even in the past, there have been pro-pocket and anti-pocket times: regency clothing was very slim and favored a tiny little purse called a reticule over pockets. but victorian walking skirts favored HUGE pockets because the skirts were, well, huge.
so seeing that women already have the option of having pockets or not, maybe we should mirror the question: why are men FORCED to use pockets? why don't men get to make the choice of using a bag or purse?
we know the answer, it's homophobia because how we carry things has been gendered heavily. a man wanting to fulfill the masculine gender role has no choice but to use pockets. maybe he can get away with a backpack or a messenger bag, but both are kinda large and unwieldly. men aren't allowed to use smaller purses. so men will not buy pants with small pockets, because they have no choice but to use large pockets.
conclusion: pockets are extremely gendered and maybe that Shouldn't Be A Thing. However, the woman's pocket situation is more complicated than "purse companies and clothing companies are in bed with each other" and discounts many women's actual preferences