Because that's not how trips with these friends turn out. When I suggest we go our separate ways for a bit one of them gets VERY offended. The statement I heard over and over was "We came on this trip to spend time with you guys!"
If we just bumped into them at WDW all the ADRs and stuff would already be set with very little wiggle room, so that situation would be different.
Oh, baby, I have been in your shoes. We had friends (The Jones Family) who had a trip planned to WDW for over a year. They had FIRM PLANS to go with "The Smith Family," who were close friends of theirs. They had a set date. A FIRM set date. You get the picture.
DH did not want to go with the Jones Family because they don't seem to know how to say "no" to a child. We prefer children who are under at least a minimum of control.

We knew a trip with them would be a bad, bad thing. So we selected a trip date well away from theirs, planned around a special family event. Meaning, we could not change our dates, as they were tied to an event.
Knowing....or so I thought....that the Jones Family was firmly committed to vacationing with the Smith Family (and had been for over a year) I casually mentioned to Mrs. Jones that we were going to WDW. Lord help me, she says that they'd rather go with us than the Smiths!!!! She cancels the long-planned trip with the Smiths (I DID NOT SEE THAT COMING!!!) and tells me they'll come with us!
It's not as if I could stop her. Believe me, if they'd back out on the Smiths, a subtle hint would have had no impact whatsoever. I tried my best to steer them to a different resort, but nothing doing. One saving grace was that we did stay longer than them, so we had some alone time. Otherwise, they wanted to be with us every day. DH was not a happy camper. Can't say I was either.
People ask, "Why did you go on vacation with them in the first place?" Well, I'd planned OUR vacation months away from THEIR vacation that was supposed to be set in stone. How was I to know they'd ditch their other friends and make reservations to come along with us?
OP, pray for them to change their mind, or you need to change your reservation and let them be stuck with theirs. Best of luck. I feel for you.