It occurs because of what is termed Erosion of Language.
This occurred in the past, and gave us the irregularities. And you can see it happening now:
In Australia, you will hear:
I wish he had of given it to me/I wish he had of gone when I axed him.
"I dunno" instead of "I don't know."
Let some pupil in the future ask some poor teacher why 'dunno' is such an irregular verb. "Teacher, why do we say, 'I dunno' but not 'Dunno you?; or "Dun you no?" Why, in the negative, do we say, "I dunno", but in the positive, say,"I know." Is there a reason why there are irregular verbs/plural forms etc,. Miss?
In America you hear:
I had ran/went to the shop./I had sang. They are 'smoothing' out the irregularities, giving us 'sing, sang, singing' and maybe 'gone' and 'run' and 'sung' will disappear.
All around, people say things like, "The boat sunk" instead of 'sank'.
Pity the poor teachers of the future trying to explain why an instrument used for chopping wood also means 'seeking information'.
Why can't the past tense of 'go' be 'goed'?
You mean, like the past tense of 'make' used to be (circa 13th century) 'maked', pronounced 'make -ed'. This, it seems, was too much to say, so the 'k' became more and more 'swallowed' till we had 'ma'ed' - and not much of a jump then, to 'made'. So, when this took hold by the 15th century, some Victorian school ma'am then had to explain why the past tense of 'make' is the irregular 'made'
It's partly caused by slovenly speech; and it's whether the changes have any rhyme or reason. For instance, 'why do we use the preposition 'of' in the past perfect form of a verb as in 'had of given'. One day, someone in this forum will be asking!!
In all honesty, I should point out that my little rant doesn't compare with the more articulate doomsayers for the future of the English language. In 1712, Jonathan Swift (of Gulliver's Travels fame) wrote:"I do here, in the Name of all the Learned and Polite Persons of the Nation complain...that our language is extremely imperfect; that its daily Improvements are by no means in proportion to its daily Corruption..."
As recently as 1974, a Viennese critic called Weigel wrote: "Every age claims that its language is more endangered and threatened by decay than never before. In our time, however, language really is endangered and threatened by decay as never before..."
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