At some point, people realized that 90% of data science work was building pipes and keeping them clean. And then data engineering was invented so that the data scientists wouldn't have to get their hands as dirty. :)
A snappier version which gives the flavour: Data scientists use {tensor flow, scikit-learn, parquet, hadoop, spark, scrapers, etc}. Data engineers write {tensor flow, scikit-learn, parquet, hadoop, spark, scrapers, etc}.
You could also look it up...