> By far the normal case is that I can isolate it to a function or a query or a class pretty quickly

In general, this takes human-interactive time. Maybe not much, but generally more interactive time than is required to write the bisect test script and invoke `git bisect run ...`

The fact that it's noninteractive means that you can do other work in the meantime. Once it's done you might well have more information than you'd have if you had used the same time manually reducing it interactively by trying to reduce the scope of the bug.