If you want to sell your products into the EU/UK/China, then you have to abide by the laws of the EU/UK/China.
That includes the laws that stop you from buying all your competitors to become a monopoly, and then abusing that monopoly.
The merged company wants to continue selling its goods into those countries.
Also, the two companies likely have subsidiaries in the EU/UK/China, so the change of ownership of those subsidiaries is subject to EU/UK/China law.
So the merger agreement includes a clause that says the two companies will ask all the relevant authorities for permission. If the two companies can't get permission for the merger, then the merger agreement says that the merger will be cancelled.