The in app duplicate check, will only enable the View Duplicate button on the tool bar if:

1 - Email OR phone match

2 - Email OR phone match and company or contact name are similar (fuzzy search)

The duplicate check would be better if

1 - Email OR phone match OR company or contact name match (fuzzy search)

We work with parties who 'Refer' leads to us and we are sometimes not able to know the email or phone number at the lead stage. But we usually have a contact name or company name at this point.

https://learn.microsoft.com/en-us/dynamics365/sales/manage-duplicate-leads