It is because the phonetic tools (of English and speaker's native language) are too different.
The secondary reason is that in the languages of Asian region (namely, those belonging to Sino-Tibetan, Tai–Kadai, and, to a certain extent, Austroasiatic families), the syllable arguably plays a bigger role (than in English).
Without loss of generality, let me focus on Thai speakers speaking English.
While there is a broad set of initial consonants in Thai syllable, the final consonants are very limited. Namely, only voiceless plosives
[ng/m/n], and sonorants
[y/w] are available. The rest of the consonants, when in final position, are mapped to the eight ones above.
That's why you can hear
/gon/ instead of "goal" (a football term),
/dit/ instead of "this", or
/bit/ instead of "beach".
English words containing consonant clusters can also be distorted, either by "converting" consonants to syllables or by simply losing them. This is easy to notice when some Thai speakers say
-s suffix denoting the plural number; the very word "words" can be pronounced
/wot/, completely losing the final
-s consonant. Note, no vowel → no syllable → no sound.
Also, many distortion happens to R-colored vowels: "there" can convert to
/de/ and so on.
All listed phenomenons can be perceived by English speaker as chopping off the words at the end.
- If you like to help your friends mastering final consonants, consider the technique I described here. The trick is very simple: if they can not pronounce e.g.
[b] as a final consonant, make it initial!