It was a standard from a time when we had green-screen terminals only capable of displaying an 80x24 grid of characters. (https://en.wikipedia.org/wiki/VT100)
It's worth asking - does the standard make sense still, given how we edit today?
It's not just about whether your hardware is capable of displaying wider lines. If it were only about hardware capability, why wouldn't we be writing code that's 200 or 300 characters wide?
* Studies of readability generally show that it declines once lines of text are longer than 60-70 characters, not counting whitespace or punctuation. At that point, humans have difficulty finding the beginning of the next line, which slows them down. You can compensate for this by increasing line spacing but you lose a bunch of space that way. The vast majority of professionally typeset natural language material is limited to about this length, or even shorter.
* People read code in terminals. Most terminals default to 80 columns wide. Consider people that develop on multiple computers and multiple OSs, and have to reconfigure them all. Or if you use a new computer or loaner computer the defaults will be back to 80. So if you change to 120 columns, you have to do it over and over again. Same with text editors, but less so.
* Side-by-side diffs can get cumbersome if the text is more than 80 columns wide, and consider that font sizes vary, and some people like their monitors vertical for reading diffs so they can see more context. On my 24" 1920x1200 monitor, I can easily read a side-by-side 80 column diff, very nearly 100, but definitely not 120.
* As a heuristic, an abundance of wide lines often indicate problems with the code itself. Too much nesting or something like that. This depends on the language and indentation used, it's generally accepted that Java code will be something like 25% wider.
I'm not saying that 80 columns is the right choice, only that there are reasons to support that choice. Just like there are reasons to choose 100 or 120.
I exclusively program on a small laptop and don't have great eyesight. Formatting code such that it only looks good on huge displays makes my life more difficult.
It's worth asking - does the standard make sense still, given how we edit today?