Bring in a junior developer in the middle and you're gonna have to train them, slowing down the process. Bring in a new senior and he might not slow you down, but won't be productive for a while. Bring a designer and that overhead becomes even higher. Bring a second designer and it's even worse. Another stakeholder? Same deal.
Dailies and Retros are too crowded? Divide the team in two and bring another manager? Same deal.
Software is hard. Head count doesn't mean speed. In fact it often means the opposite.
EDIT: About solo devs, it's funny that in HN there is a bit of a cult for good developers that manage to make things useable for others, and that includes everything from products to libraries and languages. But as soon as someone tells a story about someone or some team being unable to take over a solo-dev work, their work is immediately treated as if it were complete garbage. One thing that made me a better developer was stopping having this attitude. Maybe the reason I can't understand is that I am the one who's not good enough.
EDIT 2: Btw, sorry if my tone sounds as if I'm disagreeing with you. I’m not! I also don't think you were doubting the OP. It's just that I keep seeing this industry making the same bad assumptions about team-size, year in year out.
'More' meaning in every non-technical context. More paper, more communication, more heads to count, etc.
I worked in a Softbank startup that start hiring like crazy for positions that weren't necessary, because that's what Softbank asked for. It was a crazy place to work where anything bringing productivity gains and solving developer pain wasn't rewarded. But, of course, we several people working on developer experience, because there was not enough work for all developers.
On the other hand, maybe this is a good thing. It means more jobs for developers. More money redistribution. But on the other hand it is soul crushing to see something that could be a 2-hour walk on the park becoming a 3-month transatlantic expedition involving several teams.
In the job above, I worked for a few weeks on a team that was responsible for one single screen, and this screen had three checkboxes which we used to update some third-party API. No, there wasn't much more than that. It made me want to kill myself.
That individuals can truly develop amazing things which most teams could only hope to reach shouldn't be doubted. Plenty of solo projects with absolutely astounding results exist to prove the opposite. Enough that one could argue they fall outside the realm of 'they are geniuses with a rare talent'. Management types like to pretend developers are idiot savants capable of programming, but good developers are far more than that.