I see it this way - if I have credentials that open access to my banking via a web browser, then anyone in the world with a web browser can access it. If it's only available on a smartphone, then they have to have my phone in front of them. Most banking apps don't allow multiple installs either, so trying to replicate your login details wouldn't work, and it requires you to receive a code on your phone anyway.
It is essentially much much more secure in terms of access in my eyes. Is iOS secure? Well Apple seem to be pretty hot at keeping it that way. I trust it a lot more than I would windows - I don't have to have malware, spyware, browser blocking software running 24/7 on my phone.
I run my entire life off my phone: 100% of my finances, messaging and email, bus times, doctor appointments and prescriptions, social media, university work, calendar, order food and groceries, couple of clicks to get stuff from Amazon or Ebay, hold my phone up to the sky to check what the aircraft flying overhead is, get a G&T straight to my table, order a cab, monitor my weight/blood pressure/heart rate/blood alcohol level/blood oxygen level/hearing/toothbrushing, it holds my bus pass and my loyalty cards, stores my boarding passes and train tickets, accesses my entire music library, it can tell me what song is playing on the radio or in a bar, assists me with meditation, check the football scores, place a bet on a match, book a class at the gym, and even phone people. I never phone people though.
All that in something that fits in the palm of my hand. I do often wonder how people cope without them.