Because companies like Apple, Google, etc. treat their engineers extremely well in order to attract the best talent. Why unionize if you're payed a ton of money, have unlimited PTO and work in a very casual office environment (or from home)?
Apple employees lack class consciousness. They don’t understand that they too are working class, because they too have to sell their labor to survive. You can be pro-choice, pro-LGBTQ+, pro-DEI, etc. while still being oblivious to the class war.