Because a difficult solo encounter requires a hefty array of skills, and bringing one skill means not bringing something else. Bring Amaterasu, and you probably aren't bringing Sol. Bring Andromeda, and you don't have Joan. Reaching debuff caps (once implemented) and having overdrive gauge extension on hand means you typically have a grand total of 6 skill slots that are taken. And we haven't even talked about healing and utility skills like cleanse and dispel yet, nor the practical difficulty of assembling a group (all or mostly the same element) with the right combination of those core 6 skills.