1. Because it's most popular. Guaranteed support and "monkey see monkey do".

2. The adblocking is preconfigured, and non technical users trying to find the right extensions has a very bad history of unintentional malware. Ad block? Adblock plus? Ublock? Ublock origin? This is a great example of what floors a lot of technical folk who would be "why not just install ublock origin" and fail to understand the "why should I when I can just get Brave one and it works"

3. Most people don't use macs

Not everyone is on Mac. In fact, most people use Windows. So Safari and Ladybird are out of the question, that's two gone.

They mentioned the built-in adblock

Brave is has pre-configured as block that works on everything, also a polished sync experience.