Even though "they" is often used singularly, it's traditionally just been shorthand for "anonymous person for whom I don't know his or her gender." It's drawing from a group of multiple anonymous people. Sort of like group culpability. "Whoever stole my wallet, I hope they die" is pulling identity from the pool of the public, which contains men and women.
As soon as we know the person we're talking about, our brain assigns a gender and our pronouns naturally switch to that of the respective gender. So it's a bit of an exercise to use "they" as a singular when the person isn't anonymous, we know who they are and our brain naturally thinks of them as a certain gender.
Now, I don't have a problem with gender neutral pronouns. I speak Tagalog, which has genderless pronouns, and it works totally fine. I just don't like when people suggest that switching a word we've used millions of times in certain ways to mean something new ("they" being switched to mean "I know who you are and I'm going to refrain from using a gendered pronoun") is something really easy. It's simple, yes, but its meaning is so ingrained in us that it's tough to naturally use it in this new way on the fly.
