They could have added the option to let you do more of the story in a group (and have the encounters scale in difficulty accordingly), but it's still important that they remain soloable. Otherwise down the line new players will have a hell of a time trying to get anyone to help them with "old" content that they literally can't progress without. It happened all the time in FFXI, especially during the Chains of Promathia expansion if you weren't part of the initial rush or didn't have friends who didn't mind being dragged around and forced into level capped instances (and gear didn't scale down so you needed dedicated sets per each area), but progressing through the content was mandatory if you wanted to access the end-game content of that time.

Besides, dungeons and endgame content are still plenty group-oriented and that's where you're going to be spending most of your time anyway. Nothing else besides a few specific story and class scenarios really force you to play solo.