Maybe because some of us enjoy writing campaigns.
There is nothing wrong with writing a campaign and seeing if you can find players who want to play in it. You don't have to have the players before you start working on the campaign.
Sure, that's fine. But it is then important to remember that how many months it took to do the work is not relevant to the players, when
they didn't ask you to do that work.
Let us look again at how this thread and question was framed:
"I offered up a campaign, and this jamoke doesn't want to play it exactly as I want it! Who should give in?"
I reject the implicit question of dominance present in the initial framing of the scenario. Instead, I note that the situation in the scenario exists because the creation process didn't start with the players in mind.
Now, if you happen to have the happy situation of having a large pool of players to draw from, that may still work out for you, and that's cool. But that doesn't justify the implicit
affront some of these threads have shown at a player asking for a variation. That comes off like some cooks I know who gasp and flutter their hands when a diner asks for table salt. "That dish is as
I intended it!" they cry, "How dare they ask for a modification!"
RPGs are, ultimately, a collaborative endeavor. If you put off that collaboration, you're setting yourself up for friction late in the process, where it is harder to adapt.