1. From what I've seen, the PSD2 APIs haven't really been created with end users in mind – there are non-trivial accreditation requirements on people/entities wishing to make use of those APIs, the expectation being that only professional middlemen will dally with those APIs.
2. The PSD2 APIs don't necessarily cover the full functionality of a bank's online banking functionality.
3. While you can probably still get quite far with "just" the ability to query the current account data and recent transactions, as well as being able to initiate payments, this doesn't sidestep the bank's authorisation requirements – meaning that unless you can use a hardware TAN generator or something like that, you're still dependent on the bank's app for payment and account access authorisation.