I’m not sure about this. I don’t think Monzo are neglecting security just because they’re small &/or can get away with it. Instead they’re taking a different approach, which might seem less secure (if you’re used to having to complete painful security verification for your legacy bank) but may not be -
Monzo are liable for any fraud due to unauthorised access to your account so it’s in their interest to get this right.
Personally Touch Id is perfect for me & this is an option for iPhone users. It’s on it’s way for Android users too.
But as @anon44204028 will tell you, a PIN would be useful for some users too.