Because networking and public status is a big part of the job in high end silicon valley circles. You can get hired almost exclusively based on your follower count. You can land inter-business contracts and opportunities others can't. You can leverage your reach to launch new products successfully. The jobs of executives are largely this. It's not about having a moral stance on which social network to use, it's about having access to the most eyes. I don't blame them at all for doing what they do, it's totally sensible. The question here is kind of like asking "why does MrBeast not just spend his day building his next set??" - because public status is how it all works