It's because it's never shown to be an option.
Little girls play with mega-bloks and lego.
I don't know when I learnt that there were such jobs as bricklaying. scaffolding etc.
Growing up, the occupations I knew of were schoolmaster (m), schoolteacher (f) doctor, dentist, nurse, solicitor, accountant, vicar, mechanic, builder, electrician, plumber, engineer, architect, factory worker (m if engineering, f if clothing), shop assistant, cook (f), chef (m), dinner lady, school caretaker, newsagent, author etc
All were clearly defined as male or female. A nurse was female, a doctor was male, otherwise they were a male nurse or a lady doctor.