The thing is if you have random soloers you can't really expect to have a proper setup or many options, not to mention someone in sparks gear will probably perform worse than my lvl119 Trusts. So there is little incentive for me to waste my limited play time trying to party. For the most part the only advantage of grouping would maybe be getting it done faster and slightly safer if and only if the people playing know what they are doing, but that's about it. And you may even need to go on a lower difficulty if for some reason the people in the party are worse than your Trust. Now, I'm not discounting the possibility of bumping into well geared people you can actually get a higher difficulty Ambuscade done with and that happen to have the proper jobs for it right there, but that's something far from reliable.
Also, players soloing are usually doing so because they have very good reasons not to group unless grouping can be done quickly and painlessly on the spot. Because if you are waiting for people to change jobs and gear up or whatever you are going to lose many solo players unless they happen to have enough time to waste on that. And if you are trying a more difficult run you'd like to at least know you will be getting more for it to make it worth the extra prep time and you wont end up wasting it because the people playing are worse than Trusts. But how exactly do you know that beforehand partying with randoms? Are you willing to gamble your time for the chance of getting slightly better rewards or are you better off doing your thing and repeating the event at your own pace?
That is the question. And the main reason I personally rather skip an event than wasting time trying to party with random people whenever I have time to do the fights.
PS > As a side note that is one of the reasons why I really hated the Content Finder in XIV so much. You get grouped with a bunch of random people who may or may not know what they are doing, and maybe cause you to waste your time on a loss. Not only that but if you happen to be the inexperienced or undergeared one there is no way for you to learn the instance on your own at your own pace so you have to ruin other people's runs while you learn and/or get better gear. And let's not forget how f'ing awful it was when they had the brilliant idea of setting the mission battlefields as farmable events, so you had people b*tching at you for wanting to watch the cutscenes and just killing the monster before you were even out of it. Yep, very epic those story fights were. Woo... hoo....