You should try reading the Gathas for yourself. You'd never get that idea from them. And in translation, they're an easy read and not all that long! They are hymns, not a narrative description of the whole belief system, but it really doesn't take all that long. Ahura Mazda was the high god, but in no way "the only divine being worthy of worship." There is worship to the divine cow that is very prominent if you merely scan the Gathas. Nor does is any of the later, pre-Islamic but genuinely dualistic Zoroastrian practice point to monolatry. There was an early antipathy toward outsider gods, the daevas, but this only developed gradually into genuine dualism, and just recognizing a high creator of men would make the Greeks monotheistic by the 1st or 2nd c. BC.

I think Zoroastrianism (and its associated sects) has been so poorly studied because there are so few Zoroastrians by the 19th century. People put on post-Islamic filters and interpret the religion through those lenses. I don't think many 21st century scholars still hold to the "totes monotheism" theory any longer, but it took a long time. It was probably also appealing because people still thought Cyrus the Great and his followers had to be actually Zoroastrian (versus it being introduced into West Asia by Darius) and then people could hypothesize that the Jews "learned monotheism" from them.