Having spent time on the design and manufacture side of the fence, it all comes down to cost and value.
The assembly techniques that make a device difficult or impossible to repair are all used to lower assembly cost. This includes the tamper resistant screws as their head designs work better than standard screws when used with automatic screwdriver systems.
Only if the end consumer is willing to pay a higher price for a device that is easier to service will the manufactures have an incentive to build in serviceability.
Putting in a battery door and a connector for a battery adds a lot of cost if expectations are that less than 10% of users will keep the device long enough to want to change the battery.
If your competitors products sell for 10 or 20% less than yours, and have the same perceived quality and features, it gets very hard to justify the expense of repairable designs.
Another factor is the cost of service. While you may be willing to invest $30 in labor for a simple repair on a laptop or tablet, how much would you spend for repairs on a $200 media player?
The out-of-pocket cost of service along with how quickly devices become obsolete strongly contribute to the lack of designed in serviceability.
Lack of repair is not limited to consumer goods either. I've had test equipment that sells for several thousand dollars fail under warranty. Even then, the manufacture does not repair the equipment, they exchange it and send "repairable" units to the manufacturing plant to be repaired.
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