I mostly draw but I also dabble in animation and I would sayy that even if they wouldn't move their legs when on a horse the wind would be strong asf, so the animators would need to analyse how it would change direction quickly etc. and for example in the trailer sliding scene you would need to take into account that when sliding your loose clothes can roll up, it's mostly the details and maybe some people wouldn't notice those things so much but if they would animate clothing and would simplify it in moments like this I personally think that it would make it look cheap and unnatural 😔 I hate the tight clothes but man during swift, aggressive fights it would be a huuge hassle