For us, the primary reason "no kids" is the ideal is just because we never felt pulled toward parenthood. But beyond that, it's not just that we aren't beholden to the school schedule when we travel. We also only have to get one small hotel room, 2 airline tickets, two museum tickets, etc.
And beyond travel, a night out doesn't cost an extra $40 for a baby sitter, and doesn't get canceled last minute when a sitter cancels. And we can head in the Tokyo for the day and end up just deciding to stay. No worries about securing a sitter or who will take Timmy to soccer tomorrow or any of that. We can plan things spur of the moment, and that spontaneity is wonderful. Heck, even if our spontaneous decision is just to stay up late reading and then sleep until 11pm on a Sunday, these are things our friends with kids can't do. I am sure that for all (or at least nearly all) of them, whatever it is they get from having children and growing and shaping tiny humans more than offsets these things. For us, we don't believe they would.
Our choices aren't ideal for everyone, but I think that statement is always true no matter who is making it. But they are pretty darn close to ideal for us. I love my life! We aren't FIRE yet, and are planning a lavish-by-MMM standards retirement, which we hope will include lots of travel and other adventures and I'm thrilled at the prospect of sharing those things with my DH, and only my DH (except when we make a conscious decision to include others).