most of the ENTJs (and TJs in general) I've met don't spend a great deal thinking about specifically what they want, but they realize "whatever I want, money is going to help me get it" so they divert most of their energy toward their career only to realize 10-15 years later "I never figured out what I really want"
would you say this is common among ENTJs? (or does this have more to do with the fact that the majority of ENTJs are 3s and 8s)
I think it's not that you never figured out what you want, it's just that your priorities or goals may change.
If you reach a point where what you're doing doesn't feel right, it just means that your goals need to be revised. I don't think it's like you wake up one morning and think, Oh shit, I'm horribly off course ... more like you make small corrections along the way. Or you may decide to change direction, but that doesn't mean that you don't know what you want.
For me, I had a general idea of where I wanted to be and what I wanted to do in 5-10-15-20 years. Along the way, I was constantly re-evaluating and re-prioritizing, setting the bar higher, or changing the direction, but the ultimate goal (success in whatever I choose to do) never changed. It's not really about the money, it's about what the money represents: the freedom to be able to do what you want.
And, once I get proficient at something, I want to apply it to something else. The challenge is in starting from nothing and reaching the goal; it's in learning something new, then taking that an applying it to the next project, in becoming more competent. Once there, I'm ready for something else. It's like getting to the top of the mountain, and thinking .... OK, now what? I've used this phrase in posts before, but it best describes it: I have the drive to be constantly moving forward.