About 한국어 : why does Korean people pronounce ㅂ(b) as ㅁ(m)?

Q : Why  합니다 (I do) is not pronounce as h-a-P-n-i-d-a, but h-a-M-n-i-d-a (함니다)?

현우  샘 : In korean, if we say something like 합니다 (haPnida), we don't necessary pronounce the ending
                 letter or the ending consonant. And we don't say haP. So, for example, when you say 모자 or CAP
                 in english, In english you pronounce it like "KAEPh" you can hear a little Ph sound in the end, right?

경은  샘 : 네 (yes)..

현우  샘 : But in Korean, if you read the word, the English word in Korean, which would be CAP, Korean
                 people don't open their mouth again to pronounce the P or the B at the end. So it become KAEP. (I
                 think they mean that the Ph doesn't pronounce.) It sounds a little bit weird but, that's how Korean
                 letters are mostly pronounce.

현우  샘 : So, When I say "합" from "죄송합" (Joe-song-haP), I don't open my mouth again to say joe song
                 haPh, I don't say that. Not many people do that. Do you do that 경은 ?


경은  샘 : No.. Never..

현우  샘 : Hehe, So when you are about to pronounce the 4th letter of 죄송합니다 (me : they mean that you
                 say 합 right now), what shape does your mouth have right now?

경은  샘 : My mouth is close.

현우  샘 : It is close! So, when you wanna open your mouth again from  that close shape, and you wanna
                 pronounce 니 what does it become?

경은  샘 : Ham-ni


현우  샘 : Ham-ni. There's no room for the sound of Phi, because your mouth is shut and then open again. So,
                 haP-ni-da doesn't happen in Korean, and always what happen is haM-ni-da. So, even you see a
                 word that romanized like h-a-P-n-i-d-a, Korean people will read it as h-a-M-n-i-d-a.


현우  샘 : so, let's see 감사합니다 (thank you), depending to the person who romanized that expession, it can
                 be romanize as g-a-m-s-a-h-a-P-n-i-d-a or g-a-m-s-a-h-a-M-n-i-d-a. But still, it will pronounce as
                 g-a-m-s-a-h-a-M-n-i-d-a. Ok? So, I hope you understand that. ^^d



Source :  Ttmik teachers, 현우 샘 and 경은 샘 from lesson number 4. Transcribed by _gberocks.