Because they just aren't the same. A partner seems to have better access to actual ED code where as a 3rd party has to wait for various APIs. I can't really go into more detail because of NDAs but suffice it to say a big difference exists.
Like you said, most 3rd parties IMO are a joke. Out of the 3rd parties we know about (meaning have individual sections on the ED boards) VEAO, Leatherneck, and AvioDev will release products this year.
Both Razbam and Cortex Designs have gone completely silent and IRIS dropped out rather quickly. Maybe they will release something eventually but I think most are overwhelmed with the learning curve to develop for DCS and the expectations of the community are.
I'm looking forward to a handful of 3rd party airframes (Hawk, Typhoon to name a few) but what I really want is EDGE (meaning Nevada not 1.3.0) and the release of the EDGE SDK so 3rd parties can start making new theaters.
What I got to test of 1.3.0. prior to leaving the ED testers team, well all I'll say is the current theater looks much improved but it's still flying over the same place we have for what 10 years or so.