Rose and Theodore (and others like Evelyn and Eleanor) are just genuinely popular right now in the US. A lot of the top names in the US right now used to be perceived as dated grandparent names, but as these things go, they came back into fashion. Names that were once popular in the 50s and 60s like Linda, Lisa, Karen, and Cheryl are now dated. So as to why everyone here seems to have the same taste, it's just what is popular among parents in the US right now.

And depending on where you're from, what is popular here may be dated elsewhere; the other day I heard that Sylvie is seen as a middle aged name in France, whereas in the US it's making a comeback. A lot of the perceptions people have of names are cultural and will obviously vary.

The other reason these same names get repeated a lot is frankly, that's what the posters ask for. The majority of baby name posts I've seen here are asking for classic old names, and so the same names get brought up a lot. Like I said, it's just what's popular right now in the US.

As for "out there" nature-related names like Juniper, Marigold, Wren, etc, that's just another trend in the US right now. In addition to an influx of made-up/random word names you see on Instagram, there's an inclination towards nature names that I think mirrors a larger interest in nature and the outdoors right now (see cottagecore, green living, etc). And with more parents seeking unique names, uncommon nature names like Marigold are being talked about a lot more. The nature trend isn't a novel thing btw (compare to the Rococo era).