The M&A department of an investment bank is essentially different from the so-called M&A department of the Big Four or consulting companies. Investment banks are responsible for the promotion of the overall transaction, that is, the big housekeeper (from the perspective of the project); The so-called M&A department or transaction service department of the Big Four or consulting companies, to put it bluntly, is to hire customers to conduct some specific due diligence (optional from the perspective of the project). Because of this, there is a substantial difference between M&A departments applying for investment banks and so-called M&A departments applying for Big Four or consulting companies.
The M&A department of an entity enterprise (usually called business development) is not as great as you think. The so-called "helping China enterprises to go global" usually refers to browsing and locking some cursors in the industry where the enterprises are located, or considering some strategic expansion. When it comes to projects, investment banks should be hired.
I work in an investment bank myself. I hope the above popular explanations can give you some basic understanding.