Because someone gave the brand team authority and they can't have their brand take second place to Microsoft's or Apple's.

Branding teams are in my experience very picky about lots of stuff. For example, my employer's brand design team recently released a rule that when our logo is included alongside others logos in a list of companies involved in a partnership, it may not be included in a vertical list, only a horizontal list.

Using the standard background colour for the platform on your app when it's not one of the brand design team approved colours? Also out.

etc. etc.