Why big companies still failed to keep their best Talent?

Thanks to all comment
I thinhks, with best talent they always have plant in their career. So what do you thing position they have got in your company is a step in their plant. They will go when they think THAT OK. I think what the most important are relationship between company and they when they work at your company and their result working for company. May be other day they can help some thing to your company. life always go how it want

Regards