It is worth noting that I live in a very competitive market (Los Angeles) in a very competitive field (social media/copywriting).

This is very likely it.

It's natural to take job interviews personally but you have to remember that this doesn't necessarily reflect on you as a person. If you're having multiple interviews with a company and making it to the final round, chances are you're doing something right.

Interviews aren't a question of whether you're a qualified candidate or if you're a good fit for the job. It's whether you are the best possible fit for the job out of everyone else they're interviewing for the limited positions they have available.