Why we can't write XXXX, VX, IC, and XVV

we can't write VX because it means 10-5 and we also never write XVV because it means 10+5+5 that is 20 so we can simply write XX , IC means 100-1 that is why we never write it and we don't write XXXX because we can write XL that is 50-10 so we never write these numbers