Two reasons.
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Without hardware acceleration, real-time transcoding to h.265 is a big challenge on the CPU. Even on a reasonably high-end processor, you wouldn’t get many transcodes.
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Newer graphics hardware can handle accelerated encoding to h.265, which would surely allow for more simultaneous encodes (though I don’t know how many). Plex devs haven’t seen this feature as a priority in previous threads about the topic. You can see for yourself right here: