Back in the earliest days (probably before most of the readers here were born, around 1958., there was no distinction between testers and developers. They were all called programmers, and the best of them were chosen for our test group (ours, on Project Mercury, was the first test group that I know of).
Our test group was copied for a number of IBM Federal Systems projects, but over the years, people started having a different sort of test group. These groups were not made up of programmers, but were largely chosen because they would be cheaper than programmers. It was widely believed that any idiot could do testing. Many times I heard managers say they could train monkeys to sit around banging on keys.
Since that time, gradually over the years, more managers have come (ever so slowly) to realize that testing is a specialty that requires special people with special training and talent. We still have many “monkey-managers,” and for those managers, the role of testers has not changed much. But where professional testing is valued for itself, yes, the roles of tester and developer have become more similar (though not identical).
BTW, the role of monkeys hasn’t changed much, but, then, some developers play that role very well.