Because "brain chip that reads your mind to help with depression/bipolar disorder" is barely a hop, skip and a jump from all sorts of fun dystopian nightmares. On the tame end, something that already happens, is the makers of implantable devices going out of business; and then you have unsupported, degrading medical hardware implanted inside you that, if it malfunctions or breaks, you're just stuck with and can't get it fixed. Bad enough when that malfunction is with a vision- or hearing-related implant, but a brain chip could fuck up all sorts of things if it goes wrong.

On the more fanciful, yet-to-be-realized end of possible terrifying outcomes, the sky's the limit -- mind-reading, malware, rudimentary mind-control, the list goes on. I realize that sounds very sci-fi and like I'm a paranoid crackpot, but this is actually far more viable than you might believe at first. If they're actively already trying to make them effective at treating mental disorders, if they are indeed effective, then that doesn't require much further tweaking to go from treating them to causing them.

Truthfully, BCIs have the potential to be so dangerous that they either should not be developed or should only be developed with open standards and user control in mind. And absolutely not developed by for-profit entertainment companies.