>... constructing a particular set of circumstances which will never occur in the real-world system.

What do you find unlikely about a scenario where a test uses an app in a realistic way (e.g. with a browser) set up in a realistic context (e.g. with some fixed sample data) to reproduce a realistic scenario (e.g. a bug that already happened)?

I wouldn't say that isolation and realism are completely orthogonal but I find that well engineered integration tests are usually able to reproduce 90% of bugs sourced from production while unit tests can often manage only 10 or 15%. Bug in the SQL? Browser is involved? No can do.