Well, main reason is the NUC is available, while the RPi's are non-existent (locally, at least, and where available online, they're overpriced, at least from what I've found).
For the 7th gen, yes, I set that as the oldest generation processor due to H265 support (nearly all of my movies are encoded H265 using Handbrake). I don't recall if that started in the 6th gen or 7th gen, but it's definitely in the 7th gen, so that's where I set it. Initially, it was an experiment, in part due to the unavailability of the Pis, but after setting it up, overall, it seems vastly superior. The Pi4's do work, for sure, I've been using RPis for a long time in this capacity, but although it's hard to definitively quantify, the video playback on the NUC seems to be smoother. In certain situations, the Pi's seem a tad jerky. I did find one oddity on the NUC that I'll have to post a separate thread on though.
Of course I wouldn't spend over $300 for a single NUC, and that's part of the reason why the RPis were attractive (cost), but I was able to get this NUC for $80, which is about what Raspberry Pi 4's are listed for on Amazon, and the Amazon pricing is at least 50% more than they should be selling for.