Because you'll need to back and forth between your itinerary and your costings.
Once you've made your list of desired activities and tallied up the number of days needed, it's time to consider the budget (and perhaps how many days of leave you can get). At this point, you actually need to engage with step #5 as well, which is about costing everything. Because you'll need to back and forth between your itinerary and your costings. As you find out prices and fees, you'll likely need to remove (or perhaps add!) some items from your itinerary.