It is patently false that digraphs are the result of an alphabetic inventory insufficient to the task of spelling English phonemes. That would fail to explain the following digraphs, all of which have a default phone name that can also be spelled with a single letter: 1/

<ck, ch, dh, ee, ea, ei, ey, ph, gh, gu, ie, mb, kn, gn, oa, oe, oo, ps, we, rh, sh, ui, ue, wh (for some speakers)> at least and probably more. English orthography has about twice as many diagraphs as single letters, so clearly this explanation is a scientific, but... 2/

... that doesn't stop people from repeating it ad nauseam. Digraphs, like all graphemes, are driven by etymology and place value, not by some imagined failure of the alphabet. Phonics understands that <ph> is etymologically driven, but...3/

... absurdly imagines that <ph> is somehow the only grapheme out of 80-some that is thusly formulated. I find it especially absurd that phombies insist that there are 44 phonemes in English (whose English? Is there a cot~caught merger? Wet~whet? Etc.) -- but... 4/

...they can't count actual graphemes to save a life. Phonemes are abstract, and all in your head. When it comes to phonemes and graphemes, one of them can actually be inventoried and counted based on visual evidence, and one can't. /

*default phoneme