Even though we earn good wages, its relatively easy to see that workers are not often reaping the benefits of surplus value and have no ability to weigh in on determinations of how that value is allocated. More importantly, the advantages of a union are not limited to being able to negotiate a better salary, there are lots of workplace conditions that can be pretty oppressive in the tech industry, and unions offer the ability to improve all of them.
People see the sticker price of things and do some napkin math, but I'd be willing to bet in most cases, if that employee were to make the product on their own (with no help or resources or existing customers), they would not be able to make the same amount of money because they would have to face "new company" risks and statistics. That is pretty bleak.