I'm not sure they should be assuming that you're going to install Bazzite with a windows partition still in place at all. So it's less confusing and more streamlined (and I suppose novice friendly) to not include that. It's more complicated to do the partitioning process (which their installer also doesn't do and does provide a means for — gparted isn't included in their installer for instance) so you're relying on windows or other tools to even partition the drive in the first place.

Their tool is setup the way it is to make installing only Bazzite, and I'm not sure that adding extra options to it is going to make it more usable and understandable for someone who's just starting out in Linux but can if the added utility is there.

The tool doesn't give you options to really amend your install after the fact, add or remove features etc. But should it?

Edit: For ~~charities~~ clarity's sake I'm going to point out that if you make a partition for it EFI boot you can use their tool to set that up but it involves going into advanced features and you still have to create those partitions (Linux partition separate from windows and EFI boot separate from windows EFI boot etc) in order to do so. If you do select advanced options you will get more information but it's been awhile since I did this and I don't remember specifics. I'll check the guide probably sometime today and try to report back.