because: This image (or other media file) is in the public domain because its copyright has expired.
This applies to the United States, where Works published prior to 1978 were copyright protected for a maximum of 75 years. See Circular 1 "COPYRIGHT BASICS" PDF . Works published before 1923 (in this case ca. 1620) are now in the public domain and also in countries that figure copyright from the date of death of the artist (post mortem auctoris, in this case Paul van Somer ca. 1576(1576) - 1621/1622) and that most commonly run for a period of 50 to 70 years from that date.