I think this is because a global toast service is trivial to implement, one service class / event listener, one UI component. It takes one ticket to make, and then it's just a matter of implementing the event publishes // serviceclass calls. This is much faster than implementing a plurality of ways to indicate loading and resulting success/fail.

In other words, it's a crutch that is often taken when there isn't enough budget/resources to make a proper UI (Or enough care/love/interest/skill).

I have definitely myself gone down the quick path of implementing only server side validation + toast service for projects where the customer just does not have the budget to do things entirely properly.