The subscription revenues is a decent chunk of your lifetime value (LTV) as a customer, but it's not all of it. The goal here is to squeeze as much value from you aside from that as possible, measured mostly by two things, really: the direct ad revenue, measured by dollars that go on the balance sheet, and the indirect "engagement" value measured by the KPIs (think daily, weekly, monthly active users) that go into the quarterlies. The more time you spend on the platform, the more "things" you have got used to interacting with (aka day-to-day, week-to-week "retention"), the more they can potentially "sell" to you -- and it's not just ads / youtube subscription upsells, it can be and often is other "products" on the same platform: their music streaming, their search, their documents and emails, maps, drive, etc. etc. And it just so happens that the short format is _really, really_ engaging for many folks.
The more time you spend in the mall, the fuller are the bags on the way out, be it out of chance, habit, or convenience.
The more time you spend in the mall, the fuller are the bags on the way out, be it out of chance, habit, or convenience.