Why do even prominent institutions hire publishing houses (Springer / Elsevier etc.) for their journals?
It's not trivial to run a journal. You need so many things:


A journal management system
A desk editor to constantly keep in contact with the editorial board, answer author queries, etc
A content editor familiar with the subject matter to identify topics for invited reviews, perform editorial screening, etc
A typesetter who can set every manuscript, no matter what format it's written in (Word, LaTeX, etc) into the journal's style
A copyeditor and proofreader to check the paper's English
A marketing executive to promote the journal, e.g. by finding related conferences and getting insertions
An epub executive to handle the journal's website, epub files and distribution, deal with copyright infringement, and so on
A sales executive to handle subscriptions, payments, and customer service
An artist to design the journal cover, its promotional materials, and so on
Someone to liaise with printers to print hardcopies of the journal, and then get them delivered to conferences / subscribers


Note this list covers a wide range of skills. It's unlikely a single person can do all of them without extensive training. But publishers already have people who specialize in all of these. By engaging Springer / Elsevier etc to handle their journal, the Institution of Engineers saves themselves the trouble of having to learn how to publish a journal. In this, it's not that different from trusting one's university's IT technicians to keep the IT network up and running. One could learn how to maintain a network and do-it-yourself, but why not just leave it to the professionals.