I'm trying to book BNE-LAX (with other connecting fights before and after, but they aren't relevant to this issue) and AA.com won't allow me to book it - the website will only route me through SYD. AA has a codeshare on the BNE-LAX flight (AA 7380). Any ideas why?

FWIW, the full itinerary is MEL-BNE-LAX-MIA. That said, the issue is the same just trying to book BNE-LAX.

Cash ticket?
Guess you want AA flight numbers for earning to the AA ffp
What class?
Is this on 1 day - 1 continuous trip or with stop overs?
Will it let you book BNE-LAX-MIA with the AA codeshare?
I think, but not 100% sure, that AA needs a published fare before it can sell.

OP's other threads. Seems to be the trip
Domestic Leg of International Itinerary
Bag Check and Allowance - MEL-SYD-LAX-ORD

Interesting. Anything on google flights or on aa.com forces either on QF flight from BNE to LAX or on AA codeshare BNE-SYD and then AA metal SYD-LAX

Might be a published fare issue or might be a restriction on the codeshare that has a codeshare number but not actually codesharing.

For example, from a quick look at the fare rules on ExpertFlyer, at least one of the BNE-LAX fares sold by AA needs:

THE FARE COMPONENT MUST INCLUDE TRAVEL VIA EACH
TRANSPACIFIC SECTOR ON
ONE OR MORE OF THE FOLLOWING
ANY AA FLIGHT OPERATED BY AA

So, the codeshare flight wouldn't qualify.

Maybe associated with this --->https://www.qantasnewsroom.com.au/me...ership-update/
But is does say codeshare's on non over lapping routes continue

I was able to book over the phone as a full J fare using AA flight number for the long haul segment (3x EQM gets me to EXP with no MR, so worth the additional cost).

Discounted business only available with the MEL-LAX leg, but I did not want to fly in the A380 again - that business class was not very comfortable and had no room to work. Hopefully their refurb program goes as planned next year.

I was changing a previously booked return MEL-SYD-LAX-MIA in I as I need to leave later than originally planned (I went through SYD originally to enjoy the QF lounge there for a few hours as well as avoid the A380, but the AA 72 SYD-LAX flight was full in J).

It is a cash ticket - no award.

Thanks everyone.

EDITED TO ADD: It was odd that the AAgent could only book me in 12F even though 12K was open. I asked for 12K and she said it wasn't open. As soon as I hung up, I went to the QF site and selected 12K.