This comes up all the time.
The side quests are not intended to be an alternate leveling patch. They are there to add additional lore for each zone. Those who like getting the side lore will do the quests (and the number is more than you seem to think). The XP reflects the minimal effort required to complete those quests.
Those who aren't interested in the side lore don't have to do the quests. They're completely optional. By keeping the XP earned low, no one in the position of feeling like they're mandatory to do.
As you point out, FATEs are still available for those looking to level in open world content. Join a good FATE group and you'll get about a level of experience every 40-50 minutes in zones appropriate for your level.
Hunt bills are another option. Completing a set of 3 DT hunt bills rewards approximately 6.5 million XP each day you do them (varies slightly depending on your job's actual level) and shouldn't take more than 30 minutes to complete (most of that being travel time).
You did not get that much experience from Shadowbringers side quests. We get this same complaint every expansion and Shadowbringers was no exception. Shadowbringers just happened to be the first time that XP on the side quests scaled to job level. The XP earned was still substandard in the eyes of players looking for easy ways to get additional XP. Here's an old reddit post to read on that very subject:
https://www.reddit.com/r/ffxiv/comme...ide_quest_exp/