I don't know if "don't count" is the right word... but my group doesn't use player side 3pp products... so no classes for 3pp.I imagine they probably think the same thing about why players are so cold about using new classes that other third-parties make. Why are they the only company that can make a Psionicist that players will accept, when there have been plenty of Psionicists made by other people across the 'net to choose from and use? And if players want a Warlord or the infamous "arcane half-caster"... you can find all manner of versions out there as well. But apparently those don't count?
Here is why. back in 3.5 (and a bit into 4e) we did, and what we found what happened went something like this... 1 player would find a cool class, or race or feat or magic item... anything really, and bring it to the DM and say "Can I use X" and the DM would read through it and give a thumbs up or thumbs down... we have plenty of DMs so if 1 said no you could try 2-5 more people depending on the year in question...
As we got older the less time we had to dedicate to gaming things. So today the idea that I as a DM would keep up with talk and read about all the WotC books is hard on me along with school work and wedding planning... and for Jon that is 2 jobs (although 1 just like 30 hrs a month) and being married and having a kid, and for matt that has gone from working at a store to owning a store... and we ALL need more 'down' time.
So, If I grab a warlord I like and Jon grabs a half arcane caster he likes and Linda grabs a complex warrior she likes and Kurt is nice... he just has 3 spells he wants his cleric to have from these 2 books.... that is now 4 books for matt to review,
now remember there are atleast 4 warlords on DMsguild... so If I am the DM and Ross brings me 1, and I don;t like it, I used to just go look for 1 I did and say "how about this?" I don't have the time to look them all over.
Your right they can't... unless (and this may sound crazy) they put out all of the classes they had in all teh PHB1s through out the games history that would be a great start... #JusticeforWarlordAt some point WotC probably just threw up their hands and said "We went through all the effort to use the OGL for 5E so that players could get everything they wanted out of D&D that we ourselves were not interested or able to make... and yet players don't want to any of it and keep banging the drum for us to make it. We just can't win here!"