In my younger ( and single ) days I would usually travel alone which afforded me the flexibility to alter my plans on a whim. I gravitate more towards museums and have happily spend an entire day at the Hermitage, British Museum, etc. There were trips where I had others with me but there was no expectation that we would stick together.
For vacations where I’m actually doing sightseeing (not just staying with a friend for a couple of days to catch up), I much prefer to be alone. I guess an exception would be if I had a friend who lived in some major sightseeing location – if I knew someone in e.g. Paris then I’d probably hope they could show me around a little bit.
As the OP says, I like being able to see what I like at my own pace, eat when I’m hungry and without having to debate about where we should go, and have no one to apologize to if something I wanted to try doesn’t work out well. I did once have tentative plans to make a trip to Europe with a friend (she had to back out due to the cost), but this was a specific person who I knew would be fine with either sticking with me if she wanted to do what I had in mind for the day or going her own way if she’d rather do something else.
Two years ago I did make a trip to New York with my mother and (adult) sister, and this only served to remind me how much I prefer to travel alone. My mother is fine, but my sister turned out to be basically my nightmare travel companion, someone who refused to make plans in advance but at the same time wanted to dictate everything we did. I’m not in a hurry to repeat that experience.