Modders have the luxury of infinite time, no deadlines and not being on someone's payroll who can veto their work and tell them to re-do it at any given moment. On top of the fact that they aren't being forced to also make hundreds to thousands of other 3D models every 3-4 months with strict deadlines to get it all done.
It's extremely disingenuous to compare the two of them. Speaking as someone who's had game dev experience (albeit not in 3D modelling itself, but programming.) It's almost 100% guaranteed for such a massive project like FF14, they have extremely set in stone scheduling & targets laid out literally months or years in advance. Let's take a look at the large amount of work the 3D modellers have to do each patch cycle.
For even patches:
-The design of 4 raid bosses, all of which include the 3D modelling & meshing of their arenas, animations for said arenas, every single animation for the boss, adds that appear in the boss fights, animations for all said adds, alternate versions of the arena in the case that the arena changes mid-fight (E1s, E4s, E6s, E8s, E9s, E12s). Models for all the new raid gear, weapons for said raid, including for the raid minion & mount.
For odd patches:
-The modelling & meshing for entirely new zones for relic content (Eureka, Bozja, etc.), which include the models & animations for every new enemy, boss, etc. Models for all the new weapons & armor, including vfx animation rigging for their glowing effects (Relic weapon + Zone-flavored gear like Elemental & bozjan gear). Not to mention they have to do some work for Ultimates, rigging up new animations, textures, and in TEA's case, entirely new models.
For all patches:
-Animation rigging for all new animations that are required for cutscenes from older characters. Models + animations for every new trial + EX version. Meshing + Modelling of every new dungeon that is released. Modelling + Animations for every new monster that is added. Modelling for all the new dungeon gear. Modelling for minions/mounts/gear/etc that is added in each patch.
In total, they make thousands of models every 3.5 months as dictated by their schedule.
Stick any Modder into that environment and they will crumble. Modders don't have to worry about time constraints and deadlines. They don't have to worry about a boss vetoing their idea because they have priorities to get done. They aren't trying to also make literally hundreds of models that are put onto their plate simultaneously. Plus, Modders already have 90% of the work done for them: the game already has the racial attributes & scaling done for them, it already has bone weighting done for them in many cases; neither do they have to worry about programmers also needing to take time to add the newly created items into the databases and setup all the coding links needed to make it work in-game.
It's easy to put a single hat on a Hrothgar by a modder. Ask that modder to make everyheadpiece compatible and see how far they get. Oh, and not just for Hrothgar, but for Viera too. Because there's zero way Square'll release any headpieces unless they complete both's missing libraries to prevent people from complaining about it further. ("but why are they only half finished? I wanted <x> headpiece, not <y>, lazy devs", etc.)
Fixing Hrothgar & Viera isn't an issue of money. It's an issue of time they don't have. When they have to make thousands of models each patch on tight deadlines, they're not touching those two races until 6.0 development when they have much, much more time to get things done.
As far as why they released them that way? who knows. They could have been testing the water. Perhaps they ran out of time and didn't want to say 'oops, races delayed to 6.0 after we announced them and showed them off, teehee.' because you know how well that would have gone with quite a vocal number of the playerbase. (Read: players who still are grumpy over quotes said in 2013-2014 that were broken due to unfeasible development like housing & FC primal summoning and hold it against the devs to this day, hence why the devs have long since adopted the strategy of 'unless it's 95% finished, don't talk about it in interviews, and dismiss any questions with 'we'll look into it'/'please look forward to it'/'maybe'.) They did warn well in advance that the new races wouldn't be compatible with headgear at all. it's thanks to the 3D modelling team taking hundreds of hours out of their free time at home that they could have used doing anything else that the two races even got glasses/small headpieces like that to work in time for SHB launch.