What are the reasons for people writing in English with imperfect grammar and spelling?

Well, let’s start with spelling. English spelling doesn’t make sense. Although other languages, like French, also have spelling where one letter stands for a different sound (e.g. “oiseaux” is pronounced “wazoh”) at least their spellings are consistent for each sound (that “’-eaux” always has the same pronunciation). English spelling is difficult to master and I still have problems with common words like occasionally (got it right!) and parallel (got that right too!). However, I often put an “ss” in the first and a “rr” in the second.

And grammar is no picnic either. One of the hardest issues is “subject verb agreement”. Either can be singular or plural, and you have to match them so:

I was (singular singular)

They were (plural plural)

The problem happens when you put an interceding phrase between the subject and the verb. The verb has to still match the subject noun and not the nearest noun, which is the tendency.

You also have grammatical doublets and triplets that are pronounced the same but spelled differently:

To, too, two

There, their, they’re

Your, you’re

Its, it’s

And lastly, doublets that mean the same thing but have to be used in a grammatically proper sense

Fewer, less

Many, more

Can, may

And we constantly ignore the rules

But then we get to the issue of pedantry - correcting a perfectly understandable phrase because it’s not entirely correct.

“To boldly go where no-one has gone before”

Split infinitive! Actually, there’s no such rule and whatever rule there was was from Latin where the infinitive is always a word, not a phrase.

“Can I go to the bathroom? I don’t know CAN you?

Yeah, you’re supposed to use “may” but whatever.

“Ain’t isn’t a proper word, look it up!”, “I can’t, I ain’t got a dictionary”.

Ain’t entered American English from Black Vernacular English, where it’s entirely correct.