MS was trying to pivot away from their 90s platforms, and Borland was a potential destination for customers jumping ship from stuff like VB.

It was a different time. Even dinosaurs like IBM were still competitive in some verticals.

Because the objective of Microsoft's recruitment was not just to acquire talent, it was to diminish their leading competitor.

In a nutshell: Because Borland, more than anyone else, had hugely superior development tools (compilers and IDEs) for Windows.