The VOA has a discussion why, with different opinions from different Zim leaders.
my opinion? it's not the sanctions (which can be ignored easily by using fronts and proxies). It's because they are worried they will lose their investment. Either Mugabe will decide the company belongs to "the people" (aka his cronies) or officials will want excess kickbacks (although if this is not excess, it's usually written off as business expenses). But they also worry a war could destroy their investments.
Then there is the problem of a crumbling infrastructure, and the fact that the hardest working, best educated Zimbabweans now are working in South Africa.
But what do I know?