You fix things that are broken.
Perfectly healthy teeth that allow you to chew properly are not broken, even if they are not as perfectly parallel as you might prefer them.
Bending perfetly healthy teeth is not a repair, it is a cosmetic surgery and not different from enlarging a perfectly healthy yet not too big female breast etc.
It is an option, not a necessity. And it is mostly an American thing. In the UK or in Germany not half as many kids wear braces than in the US becaus: Why fix something that isn't broken.
In her generation the removal of wisdom teeth because they might push the other teeth a bit together so that they loose their parallel shape just was not known in Dench's generation. My dad and many of his frieds has teeth very similar to hers and noone ever cared.

Some reading might be the order of the day.
"Americans do not have better teeth than the English. In fact, by some measures Americans' teeth are actually worse."
washingtonpost.com/news/to-your-health/wp/2015/12/16/jokes-on-us-american-teeth-are-as-bad-as-british-teeth-study-says/
Other news outlets are available and a simple google search will find more.

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british actors feel less of a need to look perfect

Really, jellyree? Most of us actually have regular dental check ups but don't bother having needless cosmetic stuff done! Icy drinks? Um.yes, we like icy drinks. Cheeseburgers? Overrated!
I was gonna let you beep me, but I got my rag, and I know how you hate a mess

As you become more experienced and worldly, you'll find that Great Britain is not into dentistry, icy drinks, or a really good cheesburger.
Thank goodness for that.
If there aren't any skeletons in a man's closet, there's probably a Bertha in his attic.

As you become more experienced and worldly, you'll find that Great Britain is not into dentistry, icy drinks, or a really good cheesburger.

Because she isn't from the US.