It makes me cringe sometimes and I have tried to read posts I genuinely cannot understand.
But you can't understand why people may do this? I think that shows a significant lack of imagination on your part OP!
Lots of people write as they hear words, because they don't read much or what they do read is texts/posts written by people who also aren't great at literacy.
Can't sounds like 'carn't' in some accents.
Where and were sound the same in some (yup, go to North Manchester, they both sound like 'wurr' - 'wurrs the milk' 'we wurr goin' to see wurr t'murder 'appened'.. etc.
It's the same reason you see people write 'spag bowl' or 'chester draws' - if you've never seen it written down correctly, or spoken without a thick accent - you might well write it that way.
On a regular basis I see 'we was' and 'should of' and 'ano' (I know) and as it works for people, there's no reason for them to change that, no one changes behaviour that works!