I spent the last 3 years selling access-control/paywall-like software to large news agencies, and in the process, met with top executives in almost every major news organization/conglomerate in the US, Europe, and AUNZ.
This is absolutely the number 1 reason the major news organizations don't want to bundle up, and in the case of the top 5 (NYT, WaPo, FT, Guardian), works. Interestingly the second reason is brand. Nobody wants brand dilution from being considered in a "pool" with other publications, because it will reduce their power to charge more for their product in ads and subscription. While small publications actually do want solutions like this, generally the large publications with readership are even more upset to be lumped in with the small publications than they are to be lumped in with their competitors.
One solution which I saw but wasn't worth my company's time to explore was finding a bundle of niche publications willing to work together on a shared content-for-pay solution and had a loyal readership willing to pay, and growing outwards from that seed. But that's a very uphill battle.