Since you have asked us as strangers to speculate (and that's all we can do, since as others have said, we don't know you, don't know your spouse, don't know if you have kids, don't know your personalities, don't know your parents or their personalities, don't know what it is like when you all 'do' get together, etc...........with all those disclaimers, I'll make some guesses:
1) Your parents have worked hard their entire lives...working and/or raising you and your siblings. They (maybe) have some peace in their lives, and it is more 'peaceful' to travel without all the family drama (that likely enters anytime the whole group gets together).
2) Do you know the meaning of 'vacation getaway'? Not only is it an opportunity to get away from one's normal location, but also all the day-to-days of that location. A chance to meet new people, experience new things, make new decisions, etc. If you all are along for the ride, it's just the same ole same ole, but in a different location.
3) With two people (or even four when the other couple traveled with them), they can be more spontaneous.....they can decide instantly to go to this French cafe or that one. With a large group of people (of various ages), it is harder to get a large table, decide if there is something on the menu that little Joey would like, or hope that son-in-law Steve does not over drink again.
They can also decide to get breakfast via room service without having to explain 'why they didn't want to meet the whole clan for breakfast, etc.
They can hop in a cab and go instantly to see a site, without having to collect everyone, their water bottles, etc.
There is no 'whining,' of 'I'm tired', 'I hate museums,' 'I'm hot/I'm cold,' 'gosh not more stairs,' etc, etc., etc.
4) Maybe they want a little romance....just a chance to have a quiet dinner, a quiet walk in a lovely village on a full moon night, or just a chance to say to each other, 'remember when.......' Maybe travel is their reward after all the years of helping the kids get thru school, doing laundry, playing referee, etc.
5) Maybe they want to do some things without your judgment.....go into a coffee shop in Amsterdam.....go into a public bath as the locals do, etc.
........................and I write that as someone who has no children and could not even imagine what it would be like to go on a trip with an entire family.
I see what appears to be 3 and 4 generations on vacation, sometimes with the brightly-colored Smith Family T-Shirt (so they can round up everyone in a crowd quickly), and I smile and often comment to them how wonderful it is to have such a lovely family and to be creating memories together. But, at the same time, I'm also glad I'm not herding the cats!!!
Take a note: It isn't always about YOU. Sometimes it's about THEM. Be glad they are healthy, happy, and independent. Hopefully they have raised you to be that way.
Many have said 'it's their loss' about your parents.........maybe....but maybe it is also their GAIN!!
Again, none of us has enough info to do more than speculate. A licensed therapist could help you more, if it is really bugging you............and it likely is since you have turned to us to help you.
May you find peace, acceptance, and happiness.
There are many who would likely be happy to 'trade' parents with you!!!!!