is there really a bank that allows using wero from a browser?

yes, its working from within some banking apps, but it’s still apps on a mobile device, not websites on a pc (?)

why is wero only available as an app? what is the technical reason wero is not available from my online banking account or a wero website?

What you’ll need

• A compatible smartphone or tablet

https://support.wero-wallet.eu/hc/en-us/articles/25599074240401-What-devices-support-the-Wero-app