Modern PCs all conform to a standard architecture, so even though they have completely different components they all work together in the same way.
Consoles can completely change their architecture between generations, so software made for one will be completely inoperable on another. Backwards compatibility on consoles generally requires dedicated emulation software and/or legacy hardware, which bumps up the manufacturing costs.
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"I'm Commander Shepard, and you're standing in the biggest library in Citadel Space. Look me up."