Why Riot still didn't updated Karthus' and Gangplank's splash arts (from their skins)?

Karthus received a visual update 4 years ago, Gangplank received a visual and gameplay update 3 years ago. Neither of them have received updates to the splash arts from the skins they had before their visual upgrades were released. This was understandable at time because back then, Riot used to not create new splash arts for skins when they visually upgraded champions. Nowadays, however, they do. They even did new splash arts for champions that received visual upgrades but didn't had their skin splash arts updated, such as Tristana, Sion, and Twitch.

However, Gangplank and Karthus are yet to receive their updated splash arts. Some of those splash arts are pretty outdated. For example, Grim Reaper Karthus' splash art is virtually obsolete as it shows him with a skeleton face and a simple robe instead of his new lich face, his pauldrons, his belt, and his new robe. Phantom Karthus and Statue of Karthus still show him with a blackened face (and outdated outfits). In the case of Gangplank, his Sailor skin still shows him with hair, when he is bald in this skin now, and his hat is smaller.

This issue isn't that big, but some of those splash arts do not really represent the actual visual of the skins in-game. This can possibly discourage some people from purchasing those skins.

Note that skins they received after their VU have splash arts which are accurate with the VU and actually represent the skin's visual in game. Those include Fnatic Karthus, Karthus Lightbane, Captain Gangplank, Dreadnova Gangplank, and Pool Party Gangplank. Pentakill Karthus also received a new splash art, but this is mostly due to the Pentakill event that changed the splash art from every Pentakill skin existing back then.

This would also be a perfect opportunity to bring Toy Soldier Gangplank in line with other skins that have a similar theme, such as Renektoy, Pug'Maw, and more recently, Grungy Nunu.