Javascript Alert Box

Good day,

The javascript alert box always shows up at the top of the page and it prevents any other interaction with the page until it is closed by clicking the OK button or pressing ESC. Because of that, I use it give the user messages when something significant happens, like a payment failing.

My boss might want to change it to a modal that shows up in the center of the screen. She says she doesn't notice the alert message. I don't want to do this because it means more work for a simple dialog. Our CSS guy is busy with something else, our JS guy is busy with something else.

Are there UX theories as to why it shows up on top of the page?