It seems a shame to strip a show of those extras.
I would venture a guess that it would almost be less work for the studio rereleasing a title to retain those already present features than to remove them. I assume that there is some other, more nefarious reasons for their absence; such as the license purchased by the rereleasing studio doesn't include the extras because they were originally made by the studio selling the license. I've often gotten the vibe that there may be enmity between some studios. More realistically, the rereleasing studio simply didn't want to pay for the extras. It's possible that the actual content and extras are separate licenses. Why pay for the license for the special features when they can just rerelease the anime without purchasing said second license? It could also be more of a political problem. Maybe Funimation (for example) is forbidden by it's board of directors to include features/content produced by competitors such as ADV (for example). If this were the case then Funimation (for example) could
purchase the rights to an anime formally licensed by ADV (for example), but not
it's ADV (for example) special features.
Sorry 'bout the tangent. I enjoyed my flight of fancy. I hope it wasn't too tiresome for you.