I noticed this a while back, and wondered when it would change. Many years later, it still hasn’t.
When you print a stick-on label, it makes sense to cut the corners curved, as this makes it easier to remove from the backing AND harder to accidentally get peeled off after you stick it on something.
When companies (mostly charities) started printing address labels as “gifts”, they continued to cut the corner of the PAGE OF LABELS curved, but not each individual label. Some most of the address labels have four perpendicular corners, and four per sheet have three perpendicular corners and one curved corner. WHY HASN’T ANYONE FIXED THIS YET?