It's also breach of contract (which I'm labeling separately from "illegal" to avoid nitpickers, even though it could be included) due to the individual ToS on each site, which almost always include boilerplate forbidding the access of the site by "automated means" in addition to forbidding "commercial" or other non-personal use.
Before you raise the common counterarguments, please know that others have done so before you, and the courts have generally sharply disagreed with them. There is no respect for non-Google data scraping in the judiciary.