In economic terms, "transaction costs."
All that paperwork (if digitized in 2018) that we sign in getting set-up with our brokers (whether introducing or DMA) includes an ability to make a stock-lending operation work, with little more than a check-box. Thus, we all hang out under the initial broker-client umbrella. To do this for individual transactions would involve a *blizzard* of new paperwork/record-keeping/liability/exposure, etc etc etc.
So, what's to stop some well-capitalized firm from setting up a separate, "Stock-Lending" window, where that special-purpose skill/market could garner repetitive business, making the thing worthwhile for customers and suppliers alike? "Hmmmmm!"
I'll bet you that the numbers have been run, and that they get revisited every 6 months or so... Like 'fracking' -- the technology is there, it's just a matter of deciding when the development costs have inched sufficiently below the potential revenues.....