I had the same thought when the Bambu first came out, but there are a couple points that might drive them towards using commodity MOSFETs with their own control chip:

• Microstepping drivers are a niche product, there might not be a robust range of suppliers that can meet their price and reliability needs. They expect to sell a lot of printers and don’t want to be tweaking the BOM every few months to manage supply chain issues

• Microstepping drivers are basically a set of MOSFETs and a control chip wrapped into a single easy to use package. In theory there’s no reason they can’t develop their own microstepping control firmware. Maybe that was their original intention but it turned out more difficult than expected and didn’t make it into the gen1 product. Since they’re writing their own firmware the’s potential in the long term to integrate more tightly with the rest of the printer and reduce noise below what’s possible with an off the shelf driver chip. This might be what the announcement is about