Because, the 99% of people who DONT upgrade their RAM etc all have a socket or two in them at extra costs, extra weight, extra waste, etc etc etc.
I am 100% sure Apple has metrics that show how many people upgrade RAM or HDs.
I am also sure they have metrics which shows the reliability of sockets vs solder.
So,charging the 99% to have components they dont need or want so the 1% can buy a computer unsuitable to their needs and upgrade it seems stupid.
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Until you figure the extra components add risk of failure. Sockets are a higher failure point than solder.
Truth is, laptops have become so reliable that repairability is no longer a factor to consider.
Extra sockets needing extra space add to the cost of the laptop. At the University where I work we have a 4 year life cycle on laptops and 5 year on desktops. The cost savings and improved reliability means over all we see a $ saving over that period, particularly because we are also able to make a salary savings ( which allows us to buy 20-30 laptops a year from salary savings).