I’ve seen a lot of times where the actual CD doesn’t number the parts of a work, e.g., it’ll just have the track number, then “Allegro” or whatever. (Most Naxos releases seem to be like this).
It seems a lot of times people add in numbers. That seems to go against the guidelines (which say to use what’s printed on the release, and also show an example without the added numbering), but the added numbers are quite popular, so just want to make sure.
Work title: Allegro moderato
Work title: Adagio
Work title: Allegro e marcato sempre
(as actually printed on the release) vs.
Work title: 1. Allegro moderato
Work title: 2. Adagio
Work title: 3. Allegro e marcato sempre
(with numbers added—though it seems folks for some reason use an archaic number system—maybe they feel their I keys do not get enough use 
)
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We are a lot more permissive with track titles than we used to be. Back in the dark ages when works weren’t supported yet, track titles followed the work names quite strictly. That is not so important anymore, so we can stay closer to what’s on the packaging. But old habits die hard I guess.
Oh, and Roman numerals just look cool, obviously.
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Only when they’re properly chiseled in to stone 
Basically, the answer is what @mfmeulenbelt suggested - a mixture of releases added before the change, and people being used to the old way. I’ll admit I add the numbers even when not printed occasionally, although only when I know I’ll want to batch-add new works from that release, mostly because adding them to each work title one by one later is a pain and adding each work manually instead of batch-adding is also a pain.