Well, you could say that about almost any activity. Why do you play BfA? You could do something better, and run for cardio. Why do you run for cardio? You could do something better with your time, and donate your free time at a homeless shelter. Why don’t you do something more productive than helping at a homeless shelter? You could adopt an orphan. Why don’t you do something even more meaningful than adopting one orphan, and adopt five?
…You realize that this line of argument is not well thought out, no?
To answer your question: Different tastes. I hate how fast and shallow BfA plays. I want to soak in the atmosphere, I want to run on foot through the Barrens, and just meet other players, toss them a buff, a heal, just a /wave. I want to read chat while doing so, and when someone goes “Could you help me with this quest?”, I go “Sure, want me to move there?”
Logging into Classic was a bit of a culture shock, at first: Suddenly, you have to think about what’s happening around you - is the dude running into the mine with you after the same named mob? Go, ask him. maybe he’s just mining, maybe he wants that boss. Either way, he offers helping you. Bam, nice dude met, I like that.
I don’t need a gaming experience that holds my hand the entire way to end game, keeps my experience carefully guided so that nothing even remotely inconvenient may happen to me. It’s disgusting to me, I feel like a child whose parents are deathly afraid of its temper tantrums when playing BfA.
But you do you, both games are different, both are available.