Because slavery is complicated, and historically comes in a huge variety of forms, often with completely different social expectations and rights, but a general theme:
The theme is that your labor, and the value it produces, is not captured by you. You work, but do not capture the benefits of that work in meaningful and significant ways. This is directly related to "you do not own - you rent". Because that (quite literal) rent seeking behavior is leaching additional capital from you.
If you prefer, or if "slavery" is a term you feel loaded, you can substitute "serfdom" instead. It's also not quite apt - since it implies a tie to a specific geographic location, but it's close.
Alternatively, you might consider this very similar to a person in a "company town" - they work all day at the company, only to be forced to buy literally every item from the only store in town - the company's store. Which, conveniently for the company, charges them roughly their month's wage for housing/food/clothing. They are working and gaining nothing.
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So in this case, what happens when every item you need (not want) to participate in society is costing you a monthly subscription cost equivalent to your monthly wages?
What happens when internet, car, home, phone (and the apps on that phone) are all rented items, that capture literally all of your wages?
I'd actually say that for many lower class folks in the US right now this is already the case - they work 80 hours a week at two full time jobs, and literally all of it simply gets them to tomorrow. Nothing can be saved, there is no chance to "own" any of it. They certainly make plenty of value - it's just captured by someone else.
It's not going to stop there, though - it's going to keep eating through the ranks of our society. You will be free! (please note that your freedom is the choice to rent an Apple or a Google phone). You have choices! (please note that your choices are limited to renting a car from BMW/Ford/Tesla/etc). You will make decisions! (please note that your decisions are limited to selecting whether to rent content from Hulu or Netflix). You can vote for any one you want! (please note that they must be either Democrat or Republican).