In-memory databases are preferred for integration tests because: 1) They're faster than disk-based databases, 2) Can be easily reset between tests, 3) Don't require external infrastructure, 4) Provide real database behavior without persistence overhead, 5) Support concurrent test execution, 6) Allow for complete data isolation between tests, 7) Enable faster test execution compared to disk-based databases, 8) Perfect for CI/CD pipelines where test environment setup should be minimal.