Because they are so manufacturable.

When we design these things (which I do for a living) we often find we are forced into tradeoffs between repairability/recycleability and manufacturability/cost. The market wants cheaper and cheaper things. To accommodate we need to make them less repairable and recyclable.

Infortunately, this is where free market stops being a good optimizer and manual settings (laws) need to apply by requiring raiparability, which is difficult (but not completely impossible) to quantify.