The "problem" (for corporations) is that the process of signing up for a subscription is itself a major obstacle in user flow and can serve as a point where users "wake up" and realize what they are doing.

Sure, you can design your pages after the sign up to be addictive, but that wouldn't actually help you to get more subscribers - so there is not a lot of economic rationale to do so (unless you have other mechanisms to "monetize" already signed up users, such as lootboxes or in-app purchases)

In contrast, if you can use advertising to monetize non-subscribed users, you can sidestep that entire obstacle altogether. That's why there is a lot of economic incentive to design the free part of services to be addictive, as long as there is advertising.