Why? We have better alternatives, and clearly things can work without it.
It also appears to be part of POSIX.
I imagine though that ex would be the more capable alternative, and since vi is also ubiquitous, I guess that makes ex ubiquitous too. So I can see your point.
Even if you prefer vi for interactive edits, I still prefer ed for small edits.
Then there are times when your terminal is borken (or as somebody mentioned, perhaps even your keyboard is broken), this still happens more often than you think.
Recently I had to use some ajax-term thing which was just awful and completely unable to run vi properly for who knows what reason, anything that depends on curses can not be considered reliable.
But more importantly it is the standard and portable tool for scripting the editing of files.
If your terminal is broken, there are plenty of ways (reset, stty sane, whatever) to fix that. I'm also not sure why having an extra binary on my system is a good safety guard against a broken keyboard.
The ajax term case is valid I guess, though I'm not sure how common that is. I was in a case like that a year or so ago and ended up using cat and sed to get the thing back on the network.
Ed reads the entire file. If you need to move back and forth through the file, or move lines from one place to another, or reprocess the same line repeatedly, it can be far more convenient than a scripting language.
I appreciate historical respect for the tool but this falls short of showing it to be indispensable to everybody.