The Santander bikes are stolen quite frequently to facilitate crime using the bicycle (i.e. phone snatches). They are good enough for that task and have the advantage of being anonymous, and potentially hired legitimately (less likely to trigger a police stop and search).
If you use a payment card or mobile phone payment app for your travels, then the bikes are integrated, in effect.
The citymapper app/pass does quite a good job of integrating the bikes for people who don’t want to take out an annual subscription, although you have to have one of the high grade passes which includes bike credit, the basic one doesn’t. The pass is really a payment card (mastercard) with the back end rigged up to only accept requests from certain merchant IDs (TfL and other transport, including the bikes), and citymapper recharge the travel to you via your choice of payment card, at a discounted rate compared to TfL cap (but no refunds if you don’t fully get up to the pass/cap value in the week). Because the pass is a payment card, it can also be added to various TFL apps (maybe including the bike app).
I think the remaining question is why the Bikes are not integrated to the rest of TfL, i.e. if you subscribe and have the keyfob or use the bike app, why couldn’t the keyfob embed an oyster card and share an account, with autotopup, or allow the bike app to act as an oyster card (via the phone NFC payment card capability). There wouldn’t be the £300 loss charge blocker that way round, just needs a change to fob hardware/app.