1. Because westerners not have the right facial & body form
Asians standard of beauty are DEFERENT than westerners standard of beauty. We Asians born with almond shape eyes, small nose, sharp jaw, round face and smallish body form which made us Asians look cute while westerners born with deep set eyes, long nose, square jaw, square face and big wide body form which made westerners look masculine.
Related to the first reason, we cute Asians made our Anime characters based on our own Asians standard of beauty or based on our own Asians facial & body form, as a result when westerners cosplaying our Anime characters obviously made westerners look awkward - disgusting doing it. This is the reason why westerners look awkward cosplaying Anime characters while Asians look like the Anime character - better cosplaying Anime characters and not because Asians using models to Cosplay.
2. Because westerners aged too fast
Look at some of our famous Asian cosplayers such as Kipi who is her 20's but still look 13, Arisa Mizuhara who is in her 26 but still look 17 and Mei Wai who is 23 but still look 17 then look at the majority of our Anime characters who are teenager looking. This is the reason why westerners look like old nannies who trying hard to look like teenager when cosplaying Anime characters while we Asians look like babies with smooth flawless skin cosplaying Anime characters and not because Asians using photoshop.
Overall this 2 reasons are the reasons why westerners look hideous [ awkward - disgusting ] cosplaying Anime characters while Asians look better [ awesome - look like the Anime character ] cosplaying Anime characters