There was a massive incentive to solo even in the early stages of the game. Just because the exp wasn't the best it was one of the best ways to make cash while leveling a job.

Yes most jobs outside of Beastmaster or Blackmage were terrible for solo but it was part of making each class unique that some were better at some tasks than others, not this white-wash everyone can do everything we had in the beginings of XIV. Soloing was included in one of those tasks you could be awesome at or not.

You had choices for Solo exp and because you thought Party exp was better you chose to not do them, especially in the mid-years of the game it was foolish to sit around town with your flag up. You either made your own party of seeked while completing other tasks such as crafting, gathering, gardening, NM hunting, Farming, you had a near limitless supply of things you could do while waiting for a party if you didn't want to make one yourself.

Again the rewards for being in a party had to far outweigh solo exploits otherwise people would just solo because they could. Look at XIV people avoid others like a plague unless they need to because they don't need to interact with others outside of instances.