Because Asians speak different languages. Different languages have different sets of sounds.
Mandarin speakers have an L sound, but no R.
Japanese speakers have no L, and no English R. Their "R" is halfway between an R & D (some say between R & L , which is also true, but R & D indicates that the R is a flap in Japanese. R & L are not flaps in English).
Korean has a sound that sometimes sounds like L or R, depending on its environment. They are considered the same sound (speakers aren't really aware they pronounce it differently). Much like English speakers think that the H of house and the H of human are the same sound (because they use the same letter) and we only do the sound of H in human with HU as the first syllable. . If an English speaker notices, they wrongly think it's just a different kind of H.
L and R, do not exist in all languages. Sometimes one or the other; sometimes neither. Sometimes it close to one of the sounds but produced slightly differently (often making it sound like an English sound that it is close to, to English speakers who don't know the language/sound in question). Many English speakers hear a D, L, or R when they hear the Japanese "R", which is a variation on a true R and not any of those sounds.
There are many more Asian languages. Tons of Chinese languages; Vietnamese, Thai, Mongolian, Indonesian, etc.
studied phonology and linguistics; intermediate Japanese; read articles on the phonology of many other languages and on the IPA (international phonetic alphabet).