Single/dual income isn't necessarily it; if you live with a good roommate and cook at home, most of the expenses should be similar. Working and saving like a madman to shorten the runway just simplifies the risk of running into life events that throw the curveballs, like -
- Medical events will happen eventually. They are obviously far less likely from 20-30 than 20-50+
- Kids, if you want them (delaying to 30 is not unreasonable)
I only recently married, but on my own I spent around twelve years building and solidifying my career. In some hypothetical scenario where we met in college, it would actually be harder to work out career goals because we do very different work. SO's field is not lucrative so compromises would have probably resulted in less income. I still imagine difficulties keeping both careers optimized for maximum income when you have to compromise with the other.