That's a logistical impossibility.
Unless there's a champion rework in question, updating older skins for no reason other than to look better and up to par to Riot's current art standards, is just not going to happen. Why waste resources to rework older skins instead of making new and better skins for the same champions?
I understand that freshening up an older skin would might look good but let's look at this problem from a different perspective. A lot of champions have gotten a "fresh" recall animation BUT all those champions' recalls are using their already existing animations that were rarely seen. So, all those 76 champions (i think this is the number) haven't been really updated, they are just using already existing assets from when they were first made.
There's just no way a team of animators can do 76 recall animations in a few patches. This would've been a really long project to complete and would mean Riot would have to dedicate so much time for a single project that might not end up appreciated by a lot of people. And this raises the question: "Is this worth our time or are these resources better spent on something else the community might enjoy more?".
Champions are even a bigger project. VUing them without any gameplay changes would waste resources as Riot has already dedicated themselves to doing full VGUs to champions that really need them. Skins for those champions are also updated as well. Just imagine how much more work would have to be done as there are at least 3x more skins than there are champions. This wouldn't be even done in 3 years.