It doesn't happen often enough for you to rely on it for budgeting, it's a nice surprise, not something that you think of as part of your regular compensation.
(I've worked at places where your "bonus" was regular, until it wasn't: that did not go over well...)
The first startup I worked at had a year-end bonus tied to meeting all of a list of company goals. The last year I was there, the list consisted of 3 engineering goals and 1 sales goal. We had around 25 engineers and 2 salespeople.
We nailed the three engineering goals by about October. Sales didn’t hit their goal. No bonus for anyone.
You want to build animosity in a company? I can’t think of a better way than that.
(I've worked at places where your "bonus" was regular, until it wasn't: that did not go over well...)