I don't know how everyone got so obsessed with gender so quickly. Not even 10 years ago, you had emo and scene boys wearing make-up and floppy dyed hair and girls wearing shaved hair and flannel shirts, some were gay and lesbian, some were straight. Yeah, it looked stupid, teenage fashions always do, but they had fun with it and weren't stressing out over their identity or wanting to damage their bodies beyond getting some piercings that they could just take out again.
It's the same with younger kids, toy adverts used to show boys and girls playing together with the toy, or there'd be a bunch of different Lego sets that had male and female minifigs and a mix of stereotypes (like a farm set would have a pony and a tractor, but it didn't explicitly say it was for a girl or a boy). Now even Lego sets are really strictly gendered, you get Lego for boys and Lego Friends for girls. It used to be okay to be a tomboy or a sissy, or at least if it wasn't okay it was a lot more tolerated. They'd get picked on, sure, but they'd have friends and their parents would just think they were a bit weird, they wouldn't tell them their bodies were wrong or they needed Lupron.
It's a shame the autogynephilic troons have chased all the old butch lesbians and camp gay dudes out of the LGBT movement, they could teach them a few things about gender nonconformity being okay.
As for gendered Lego sets, it is because they sell more Lego that way. God forbid, a boy touches a pink Lego brick. He could catch Gay from that Lego set, especially if he makes a pink and pastel purple tower. So if there is an older sister, boy needs his own legos. If there is a younger sister, we know what to get her to complement her brother's Lego set. In essence, this is about money, sales and marketing,
It is the same with children's clothes. Manufacturers make it so that little boy pants have a very different seam line from little girl's pants. And tee shirts are pretty much made so that there is always a print on it, of a gendered cartoon character. And girls stuff is so unmistakably feminine, that it makes it socially questionable to repurpose it for a younger male sibling. The end result, for people like me and my husband, is having bins full of kid's clothes, preemie - 7, with both monster trucks or princesses indelibly stamped on them.
This is all about making money but for the end user, it amounts to complicating your life ... a lot. Especially having to take a special effort to match Frozen II-themed Christmas socks for a 3 year old, along side grinch-themed ones for boys. And summer is around the corner and we live in Southern California.
So I hope that I have addressed your question, about this uptick in assigning gender to everything.
It becomes relevant to parents, as they sort through batches of hand-me-down gendered socks and flouncy onesie blouses for newborn baby girls ... not that parents aren't aware that their daughter is a girl ...
This is my general approach to the issue. We are expecting a baby boy. We have a 5 year old boy and a 2 year old girl ... if somebody gives us hand-me downs, from here until eternity, I will accept them. But otherwise, Baby Sister can wear a gender-neutral outfit and we can pierce her ears and put a matching ribbon in her hair to not be accused of trooning her out. Unfortunately I only have Preemie and Newborn in floral prints, so he can come home in a baby blanket with cars on it or something so people don't think we are weird.
If ever invited to a baby shower, F.Y.I. Gerber makes neutral colored onesies. Children's socks come in neutral colors too. Although people might think that they want pink sequined newborn socks or whatever, when you are actually taking care of children, and not dressing dolls, a pack of neutral colored socks in two sizes too large helps you get out the door in the morning.
So personally I think that we have ushered in a new era in which we have both the leisure and the cheap manufacturing capacity to create this culture of childhood individualism. I am not that old, but when I was born (1970's) they did not have ultrasound. They also did not have Chinese sweatshops. Not everybody had consistent access to a washer and a dryer. So vintage baby clothes have a less specialized look.
Also, chances are, it does not occur to a lot of parents, to simplify their kid's wardrobes to shorts, denim or Khaki skinny-jeans, BVD's and Hanes tube socks.