The technical reason is sending a message is free.
The non-technical reason is companies prefer to keep the majority of the advertising money instead of pay users with it. Users don't complain and don't try alternative services.
> Why is this still a thing?
Because the profit is significantly higher than the cost to create/run one.
Like Nigerian email scams, it's still a thing because it still works.
Another AI war
Maybe detecting and blocking them without false positives is actually not trivial.