because you cut half the Pokémon for this reason and you have pride in your work
There are 90 Pokemon that can know tail whip. Let's say exactly half of those got cut, that's still 45 unique animations they need to make plus however many new Pokemon can learn it in gen 8. For an utterly useless move. Now let's look at just one other move that most people would probably agree has a shitty animation, Body Slam. You can see Sun and Moon's version of it
here at 13:38 and it'll probably be pretty similar in SS. That's a move that can be learned by damn near every Pokemon in the first three gens because of a move tutor in Emerald, plus a handful of later gen Pokemon. That's
hundreds. If they wanted to update the animation to something that looked nice and unique for everyone, it would require
hundreds of unique animations for a single move, even cutting 50% of the Pokemon. In a series that's approaching 800 moves. Trust me, I understand the frustration with Pokemon's lack of ambition, but some things are simply not feasible and their effort would be better directed elsewhere.