At the time Apple did this, they had already been soldering RAM onto the motherboards of iPhones because phones needed the smaller envelope. At the time, it was

At the time Apple did this, they had already been soldering RAM onto the motherboards of iPhones because phones needed the smaller envelope. At the time, it was probably more about saving money on manufacturing by not having extra, moving parts, and making it simpler to automate the fabrication. Also, even back then, this would have an impact on thermals and size, which could easily be the only justification needed.