Why is bash history substitution still enabled by default?
If you are already familiar with bash, then dealing with history substitution patterns is not much more likely to bite you than handling any other characters that are special to this shell. However, if one is unfamiliar with the shell or just never used its history substitution features, it will obviously be a surprise when seemingly innocuous unquoted or double-quoted strings triggers it.
In an interactive shell with history substitutions enabled, the ! character is special in pretty much the same way as the $ character is special, i.e. everywhere unless escaped with \ or in single-quoted strings.
As opposed to $ through, history substitutions do not expand in here-documents, and since they are line-oriented, they additionally will happen on lines where the substitution falls within an unquoted context or a double quoted context (in that line when scanned separately). See this bug report for more info.
History substitution is disabled in non-interactive shells (scripts) because the shell's command history capability is not needed there, not because the feature has "major issues". In a script, saving every command to $HISTFILE makes no sense, and history substitution likewise is not something you'd want to rely on in a script.
Whether or not it should be enabled by default or not in interactive shells can be debated (though I'm not entirely convinced that a debate here would matter much to the bash developers). You seem to think that most bash users are having problems with history expansions, but neither one of you and me know how common it is to use them.
Unix shells allow one to modify the shell's behaviour to fit one's personal needs and taste. If you want to turn off history substitutions for all your interactive shells, continue doing what you are doing with using set +H in your ~/.bashrc file, or lobby the bash developers to change the default (which, I believe, would upset and confuse more people than it would help).
History substitution is useful. Take for example
% make-me-a-sandwich
make-me-a-sandwich: Permission denied
% sudo !!
Ok.
Social/cultural inertia.
This question is in the how-humans-work problem space, so I'm going to answer from that angle, without putting forth any opinion about whether or not the feature ought to be on by default.
To start, to make sure you understand the other side, consider that the annoyance you feel about having to go out of your way to turn off the feature, is the annoyance they would feel if they had to go out of their way to turn on the feature.
Combine the above with the fact that enough bash users do use the feature, suggestions of removing it or turning it off by default are met with resistance from the people who are already comfortable with it being there by default.
Also, bash is the default shell for many people (not just in the default login or system shell sense, but in a psychological sense). If your reference frame for shell quoting is bash, if that's the shell you learned first, the fact that ! is a special shell character will feel natural and automatic to you (or at least, when you first learn it, it'll be just part of the way the shell is, just one quirk to accept among many).
And if you think about it, a lot of bash users probably encounter the history substitution syntax in a positive context: they read about it or someone shows it to them and they see the possible usefulness of it, when they're first learning bash.
It's only coming from the peripheral world of other Bourne-like shells, that you'd be bitten by ! being special and thus be inclined to view it negatively: Because if you're used to shells where you never had the feature, then your first exposure to it will be when it screws you when you're trying to get something done in a hurry.
TL;DR: Most users probably don't strongly care what the default is either way, some users like the feature and have the strong advantage of it already being that way, and there hasn't been enough people actively advocating against the feature to overcome that.