@McTag,
Quote: Slang is common (far from being rare) but it is still slang. Don't pretend you don't know the difference.
Pay McTag, a malicious know nothing, no heed, ESL/EFLs.
We find "how come" in the record of the British House of Lords.
And we find "how come" in the record of the British House of Commons.
Remember!!!!
The Longman Grammar of Written and Spoken English explains McTag's ignorance:
" [People] will all find it (the LGSWE) useful to know which grammatical patterns are common and which are rare. Hitherto this information has been based on native-speaker intuition. However, native speakers rarely have accurate perceptions of these differences: ... "