This is for me the main point. There are probably lots of people who have family, and may (more or less regularly) share their phone with their spouse or kids. In my case it’s OK for my spouse to see my account (she knows all my passwords anyway), but not for my kid! I have - on a couple of occasions - given my unlocked phone to friends whose battery had died but they needed to do a phone call.
That is quite apart from the fact that (1) my son, wife and friends all know my birthday, (2) those who know my name can easily find it on Google/Bing/etc, and (3) the dates of birth of pretty much all of South Africa is now public, (and I have no doubt that you can find most other DOBs online as well,) so the DOB is an entirely inappropriate security measure anyway.
There are two reasons why I simply cannot use Monzo for anything else than holiday spend: (1) The complete lack of ability to lock down the app from even the most simple intrusion. (2) The complete lack to do anything from my PC.