Alkyl shift vs Hydride shift in hydroboration-oxidation reaction

A part of the hydroboration oxidation reaction in alkenes is as follows:


[![enter image description here](https://i.stack.imgur.com/Fl4E3.gif)](https://i.stack.imgur.com/Fl4E3.gif)


but in the 4th step, why is alkyl shift happening instead of hydride shift (from Boron atom)?


