When you open the app on your phone, the phone probably needed to be unlocked first, though not everyone sets a PIN or biometric lock on their phone.
Then you will be prompted for a separate PIN or your fingerprint within the banking app. This provides a stronger signal to the bank that you are the legitimate account holder. If your phone is lost or stolen, the chances of someone else being able to authenticate to your banking app are considerably reduced.
With the web app, people will often save their bank website login credentials in the browser. So now someone nefarious only needs to access the unlocked computer or phone the go to the bank's website to gain unauthorised by still authenticated access to the customer's accounts.
Yes, there are now laptops and tablets with biometric authentication, so the potential is there for adding biometric authentication for banking website. However, when you look at the demographics of banking app vs website use, a strong majority of Millennials and Gen-Z do all their banking via the phone app. Gen-X also slightly favour the phone app whereas Boomers strongly prefer the website. So banks focus their development efforts where the majority of users are.