We do care about user friendliness and our user requests.
Unfortunately 1.1.2021 may be one of the many formats handled by Excel, but is not a standard date format. The expected format for importing birthdates is as Igor explained 01/01/2021 which is one of the standard date format for website databases.
We can't create an import system that can handle every format that each user will want to use depending on the software he uses, or even worse if he manually creates it.
As regards Excel, the dates can be changed in a few seconds. You need to select the column with your dates (or all cells concerned) and change the format of the cell (either right click + format cell, or it can be found in a menu). There, choose the date format needed (dd/mm/yyyy). It should automatically convert all your dates in your file.
Note that in the next AcyMailing release (7.6.1), this format will be changed to yyyy-mm-dd to improve AcyMailing custom fields handling.