I'd imagine, if there is a coherent design reason at all, its more a reflection of the specifications of differing parts of computers being in the hands of different engineering teams - eg case, motherboard and more - and there being different cost/design reasons for it.
They wouldn't be thinking about the ease of use for owners, more to do with costs and straightforward options for the manufacturing process
lots of household items have seemingly random selections of screws, even those 'security' variations on torx - I've noticed some items (some kitchen equipment for instance) have such deep recesses to get at the screws, and narrow access too, that its pretty clear it isn't meant to be maintained at home with regular tools