So, if we can define a party's goals in terms of taking on Orcus and a bunch of his minions in a particular setting, why can't players choose to define a party's goals in terms of defeating an undead army or striving to becoming a demonslayer?
You seemed to have missed a major point. We aren't defining the party's goals. We are listing goals of
individual users.
And, the reason we don't define the goals on such a high levels is that, ultimately, those high-level things are not usually the players root motivations. Exploring a continent, or growing powerful enough to take on Orcus is something we expect to take months, if not years, of play - that payoff is a long, long way off, and hopes of it are not going to keep the player at the table. We need to get at the rewards that people get every time they use the rules, not once in the campaign.
Also, if we dig down deeper, we are getting at things more basic, more applicable to a wide range of longer-term goals. I give you characters that do the sorts of things players want to do moment to moment. You get to determine what they do longer term.
Think of it this way - I'm talking about making sure someone has food and water, and you're talking more about lifestyle choices. Whatever your lifestyle, you need food, water, and air.
Like if we were asking someone out, you seem to be pitching 'let's make sure we pack all the right items in a rucksack just in case and go down to the local pub and play darts - every date'. While I'm pitching, 'lets drop everything, run away together and explore a continent'. Bet I get more dates
Apples and oranges.
I'm not designing a single date, for a specific person. I'm designing a toolkit for assisting dates, that I sell not to you in particular, but to thousands or even millions of people who go on dates. I sell you the rucksack full of stuff, and then on a particular date, you'll pull out the individual items you need to make that one date work.
You really want to go exploring a continent *without* a rucksack full of stuff on hand? I think you'll find your paramour unimpressed when he or she discovers you did drop everything - so you don't have bug spray, and you didn't so much as pack a picnic basket. Hungry and itchy does not make for a good date, you know.
Now, maybe you have worked out how to make bug repellent out of local flora, and how to forage for mangoes. But as I said, I'm not making this for you, personally and individually. I have to think about all those other people who don't have those skills, and need what's in a rucksack.