Suspect common modern practice is to string along workers on constant contracts in order to bypass time-based salary progression, especially for current generation of new workers.
Personal opinion. Middleman companies often refuse to specify exact client companies prior to contract signing, largely contract with high-turnover business with atrocious safety standards and/or workplace environment (hence the anonymous recruiting), and as a final insult extremely likely to sell personal information to spam marketers and other unsavory sorts.
Re: Contract stringalong - unsure about foreign legalities, but practice is to constantly renew short-duration contracts for targeted workers thus denying them salary increases, as they are constantly deleted-reinstated in company records as "new" employees.