Talent is in short supply.
In Silicon Valley, the common sense wisdom used to be that engineers were the most critical team members. The good ones were rare and hard to come by. Sometimes we called them ninjas or 10Xers.
More recently, the proliferation of consumer web apps made design a key differentiator. Designers became the scarce resource and salaries skyrocketed. Everyone who knew photoshop was suddenly rebranding themselves as a UX designer. No one really knew the difference between UX and UI, but we called them unicorns anyway and started the hunt.
The ideal team is often described as Hacker, Hustler & Hipster. Articles have made it into Forbes and Steve Blank posted his version back in 2011.
Even corporate teams have bought into the dogma to the degree that every innovation team must have a designer, hacker, and business person. No more than that and definitely no more than two pizzas worth of people!
The idea that there is an ideal team size and composition is...well... absurd.
(Except the pizza part. That's true from Dunbar's number.)