A quick test I ran on my Gmail client on Android wraps at approximately 40 characters on my default settings (4.3 inch screen, normal zoom), so this is a real world example of a display where a hard wrap at 66 or 78 wouldn't work all that well, with a bunch of jagged half-lines throughout the text. Throw in the number of people who have smaller screens, or use a larger font for readability, and you're essentially making emails less readable for them by adding arbitrary line breaks.

I'm a strong believer that formatting should be separated from content, and this seems to be a pretty clear-cut case of that. The basic rationale behind such a rule is for maximum flexibility for the most different types of use cases, not just yours. And I'm agreeing that I never want to see text where the wrap happens at the 100 or 150 character mark - if it's happening a lot, then the software displaying the text should be patched to reflect these use cases. Not adjusting the text itself so that it's less compatible with a wide variety of displays.

Who the heck designs a website with the intention that the reader would only maximize their window by 600 pixels or whatever?

When a web designer makes the assumption that no user would be visiting their page with 600 pixels of horizontal resolution, the resulting website generally looks like shit on phones. The functionality is also reduced when a user has to scroll horizontally.

On a side note, I have never seen a professionally typeset, printed work where the lines wrapped based on character count and not actual width of the line. I'm not talking about justified edges, I'm talking about how different combinations of letters take up different amount of space when you have non-monospaced letters and kerning and the like.

10-character string examples:

1234567890
Auctioning
Camcorders
Buccaneers
Furnishing
Backgammon
Titillates