Part of that is just because that's how I learned it.
Part of it is the proportionality he mentions: a little space between words, some more space between sentences, more space yet between paragraphs.
Like him, I also spend a fair deal of time dealing with const width fonts when all the typographers arguments go out the window.
All of this "yes, yes, I concur with this one guy on the internet!" babble is just a prelude to this:
It’s disrespectful to let writing’s constituent elements bleed into one another through imprecise demarcations. If you see me “making mistakes with comma placement”, please rest assured that I’m doing it deliberately. In most cases the comma doesn’t belong to the phrase delimited by the quotation marks that enclose it. Placing an exclamation point or question mark to the left or right of a close-quote is a weighty decision! The we violate the atomic purity of quotations with injected commas is an outrage.I do the same thing with quotes. (When I have the confidence to disregard the norm. You may have noticed it here. This gives me courage to do it more in the future. Should I receive complaints I will direct people back here.)
It makes no sense to me to interleave punctuation the way my English teachers would have me do it: the sentence starts, the quote starts, the sentence ends, then the quote ends. It's improper nesting!
I have always suspected thinking this way is highly correlated with coders. I wish I had data to back that up.