Its because these tired old ballets generate some ticket sales, nothing to do with art.
Hi Guest! Those "tired old ballets" with great branding sure put bums on seats - no question.
But I can remember the first time I saw "Giselle", from my background as a contemporary dance fan. I thought the folksy opening and the eternal triangle somewhat trite and then Giselle commits suicide and we're into that disturbing and transcendental second Act. I was taken aback and still am, although I enjoy the opening scenes more now as well.
I sometimes see "Giselle" with a VERY critical Administrator, whose main interest is contemporary. And afterwards I have to allow her some space, she says "I'm Giselle'd out" - not so much a two hanky, as a three hanky job. "Swan Lake" and sometimes "La Bayadere" can have a similar effect.
Remember that the people mounting these works have given up their lives to this art form and that these are significant works which they will think about when they look in the mirror. I think these artists take these works very seriously as art.