You can build dolls with legos. It is the act of building that is really the core that draws this line of demarcation IMO. Other stuff like whether pink is associated with masculinity is harder to judge.

Additionally don't forget practically all of our technology and science from the dawn of human kind (with many exceptions) was built from the ground up by males. That is the key, despite the exceptions, men are basically responsible for all of it. It seems illogical to dismiss the difference with a simple answer like sexism.

It may be that males in the past didn't permit women to enter STEM and that is WRONG. But there's also the fact that males built all of STEM and no parallel structure was built on the female side; that difference is worth exploring as is your remark about boys wearing pink and playing with dolls.