Is there any valid reason? Weight? Space saving?
or simply Apple don't want ppl to upgrade their own RAMs.
Less failure. Same with the display. Memory is so unlikely to be manufactured faulty that it makes financial sense to cut out the step of creating a slot for it and installing it. Just solder it straight to the board.
Less steps in manufacturing = more money in pocket of manufacturer of product.
I don't think someone at Apple is making a core decision on whether to not allow people to upgrade their laptops. I think they are making a financial decision based on economics.
Older thick LEDs used to SUCK. Then they became more reliable. Now, they are so reliable, that they are confident out of 1000 machines, 1000 parts will work. Why bother building an assembly, then screwing the LCD in? Just build a display assembly from the bare parts. It saves time and money. In the past, this made sense because many LCDs would be bad. So, the manufacturer would have to unscrew them and screw new ones in. If 999/1000 of the LCDs are good, why not just build the LCD inside the finished product?
It's like taking the time to solder a socket onto a board so you can test the performance of different ICs in the circuit. If you know that the first IC you grab is going to be the best sounding, then why not save yourself a soldering/desoldering job and solder that IC directly to the board? Same idea.