Most posts are ignored and are an absolute loss to the company. Which is why platforms like Twitter only allow you to make money from posting once you reach a certain threshold.
They're not an "absolute loss" since they cost bytes to store, and raise engagement and data metrics.
It's just that they don't want to share the fractions of pennies with everyone, so the fractions accumulate for them.
Then they pay a bit to the higher tiers, so they create the illusion that X is a parallel income source, and gives the lower tiers something to aspire to.
Carrot and stick, or rather glass beads and the hope thereof.
You actually get to generate content for the platform for free.
Without you (all of the X users), the platform would be devoid of content, just botspeak and corporate promos.
Plus, as the sibling mentioned, they monetize your visit through ads (and data use).