How so?
The manufacturer needs to work on the update.
But not without agreement with the carrier.
This is the difference between a mfr-unlocked phone, and a carrier phone.
A mfr-unlocked phone can get updates straight from the mfr--such as Google Pixel. OTOH, because it's not carrier-specific, it may lack bands/frequencies for a given carrier, or it may not have features like wifi calling, or visual voicemail, or any number of carrier-specific features. If the phone struggles on any given carrier's network, the carrier may or may not be able to support your use of that phone on their network. That's the tradeoff of a mfr-unlocked phone.
A carrier phone will have full support for all carrier features the day it's introduced, and you can get tech support for it from the carrier no matter what. But the carrier decides what they ask the mfr to do with respect to updates/upgrades.
I work in an industry not unlike this, with a product from a separate manufacturer being used as part of a larger OEM system full of OEM's gear. Said manufacturer and core product is also used by competing OEM systems. Yes, the mfr can do whatever they want--but the OEM that uses the product, determines what specific configuration of that product they want to use. The OEM also determines what upgrades/updates they want to make available.
The mfr of that particular product, being a vendor to the various OEMs, doesn't dictate to the larger OEMs what the OEMs are going to get. The OEMs specify, and the vendor supplies.
LG is in the same position. T-Mobile specifies what they want to buy, and LG sells it to them. LG does not dictate to T-Mobile what T-Mobile and its customers is going to get.
If T-Mobile decides not to buy the upgrade to Nougat, that's T-Mobile's decision as a buyer and not LG's decision as a seller. And LG doesn't sell to you and me, so LG doesn't care what you and I want in this regard. LG sells to T-Mobile (and AT&TV, etc), and what T-Mobile wants is the only thing that matters to them.
So to say "the manufacturer needs to work on the update", while technically true, isn't the story. The carrier needs to BUY the update first. Then and only then will LG work on it.
LG's business model is different than Apple's or Google's in this regard. Apple and Google have chosen to be completely independent of the carriers--but that comes with risk for the consumer.
The choice is simple: if your priority in life is instant upgrades the moment Google introduces the product, then get a Pixel phone that's outside the carrier world.