For example, a pocket singularity
sounds like a great power source, until you move it to the TYPE-MOONverse, where physics says it
can't happen, and the
setting's physics don't explain it
well enough for the World to write it off as "close enough to magecraft." They'd end up spending more energy to keep a paradigm shield running over it so that it didn't accidentally consume the
planet than they'd
ever get out of it.
D&D magic items, as a counterpoint,
have rules in place to drain them of magical energy for other uses - granted, not often
applied (depends on the edition and setting) - but they're
there. Likewise, generic items like "mana crystals," "potions," etc. can be handwaved as working
without needing a
specific magic system in place in order to make it viable.