I think this is the same as AJ Bell which I posted about earlier Electronic verification
I think it’s because they use Equifax to verify the bank account matches your name and address. Monzo seems to fail this and so they have to request confirmation that the bank belongs to use.
My employer use Equifax to verify client nominated bank accounts and it works 90% of the time. There must be a reason why it fails if you have a Monzo account. Not sure why.