^^ This/these, and also...

Hasbro/Takara-Tomy is more than capable of putting out the same level of quality releases as 3rd Party crews, but 1) they have to take into account child safety laws/regulations, and 2) HasTakTom price points. If, say, FansToys were to 'join up' with Hasbro, and some (let's say American) more-dollars-than-sense parent bought an 'adult collectible' TF for their spoilt kid, who then tried eating said collectible and choked to death, that'd be Lawsuit City for Hasbro.
If a 3rd Party outfit were to join the Official HTT Family, they'd have to both design their works to be dumb-child-proof, and (as likely as not) to make their production cost palatable for mass-release. Parts count, drop-test-survivable, moron-proof, paint apps...all of these factors would be pretty limiting to someone wanting to make high-end transforming robots for theoretically-mature, adult collectors.

Hasbro/Takara-Tomy has the design talent and resources to put out 3rd-Party-quality (and better) stuff already, but it's just not cost-effective - from a design and/or legal point of view. There's more money to be made by making mass-release, inferior-quality toys (which, to be fair, I often quite enjoy) to sell to a wider market than there is in making high-end, often-quite-delicate toys to sell to a niche market...which is what 3rd Party groups do. They don't have to worry about child-safety laws, and they don't have to worry nearly as much about product cost...they can just focus on making good toys.
In the same way as it's not in HTT's interests to 'adopt' 3rd-Party producers' works into their own portfolio, it'd also be really limiting to 3rd-Party producers to become Official.