meanwhile th, ch, sh are independent sounds that should have their own letters
Actually, Old English got had a couple letters called
þ, and
ð, or <þorn> and <eð>, which both represented what's now the <th> digraph. The only language that uses them today, however, is Icelandic.
The reason English got rid of those letters was because (1) English borrowed a lot of words from French, including their spelling, and (2) no printing press actually had those letters.
Also,
and don't really have much to do with the letters ... respectivly
Today, yes. Historically, no. <th> originated from the Latin transcription of <θ>, which at the time, was pronounced like a <th> in India and Southeast Asia (which, for the purposes here, is pronounced like the <t> as in <tea>). While theta shifted from [t
h] to [θ] in Greek, the digraph stuck around and spread, which is why we have <th> today.
As for <ch> and <sh>, they both came from Old French. <ch> was another digraph used by the Romans to represent a Greek letter, in this case, <Χ>, or <chi>, which was pronounced like <k> like in <kill> at the time. Now, it's pronounced [ x] like in German <Bach>, but that's besides the point. Old French didn't have a [k
h] sound, so it used it for a different sound that I'm not going to describe here, but
here's the wiki page for it. After shifting in pronunciation many times, it eventually became [tʃ] (for the record this is in the
IPA, not how it's actually spelled), and then just [ʃ] (pronounced like the <sh> in <shame>). This is basically the reason why adding <h> after a letter is the one of the most common ways of creating a digraph using the Latin alphabet, like <bh>, <ch>, <dh>, <gh>, <hh>, <jh>, <kh>, <lh>, <mh>, <nh>, <ph>, <qh>, <rh>, <sh>, <th>, <wh>, <xh>, and <zh>. For the record, that's an incomplete list.
TL;DR: Linguistics is complicated stuff with some semblance of reason if you squint hard enough. Also Romans.
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