I think TS wrote it because, with the exception of the 3 guys from Right Said Fred already mentioned, she's one of only 2 co-writers listed - the other being Jack A (exact surname escapes me at present!) the guy from Bleachers and FUN. Do you think she didn't write it or at least co-write?
I want to avoid going back over well trodden ground that we've discussed in exchanges before. I don't dispute that the industry likes a select group of 'names' or that they've got almost infinite resources at their disposal compared to the average bedroom producer, aspiring artist and budding songwriter. It's obvious if you look at the writers credits on pop hits that the same names keep coming up - Drake, Ed Sheeran, Charli XCX, Bonnie McKee, Bruno Mars, Julia Michaels, Dr Luke, Max Martin, Diplo, Major Lazer, Pharell, Ryan Tedder to name a few. In fairness though any TS song writer list hardly reads like a Beyonce, Riri, or Bieber number where the number of writers nearly requires a very skilled genealogist to keep on top of it. We've reached a point now where hits aren't even written by a team anymore they're actually written by TEAMS in some cases; ie teams writing in different rooms coming up with hooks, toplines etc and where the best of what is offered is put together and polished into a song.
The reason the industry heads stick to these 'names' is pretty obvious; they're a proven quantity and have a clear ability to deliver the necessary end product time after time after time,. It's hardly rocket science and I don't think the music industry is much different to any other in that it is driven by the goal of making maximum profit. Of course, in order to become one of these 'names' writers, whether those like some of the above who write for themselves and others or writers who are lesser known and write for established artists instead, have to show their worth - ie that they've the level of talent to write a hit with mass appeal. Yes the industry tends to coalesce around certain 'names' but if someone from outside that circle presents the chance for them to profit by offering them a potential hit they'll take it - why wouldn't they if it's gonna make money for them? Yes, it's hard to get into that position but not impossible -not impossible by any stretch. If you network with and/or collab with the RIGHT people your songs can reach ears you wouldn't think they can - then of course those ears want to hear more to weigh up your credentials further.