Why is herbal tea brewed at boiling temperature?

The problem is that "herbal tea" is a very broad category. Some herbs, like woody stems or roots, require a bit more "decocting," which the higher temperature helps with. Ginger or ginseng root, pine needles, rose hips, sarsaparilla, and similar plant bits all fit this bill, and some of those I'd honestly go ahead and boil for some time, not just steep. But there are other herbs that are much more delicate and benefit from lower temperatures. Lemon balm, for example. It makes the most incredible tea, but it's best to treat it like green tea with a longer steeping time. With boiling water it loses a lot of its fresh lemony taste.

Some more common herbal teas like chamomile and mint are a bit more middle-of-the road with this regard and probably respond well to a boiled steep, but you might find them better in a gentler infusion.

It's possible that two main factors are that manufacturers figure consumers can't be bothered to sort this all out and probably don't consume herbal teas consistently enough to work out these subtleties (at least relative to tea), and that the traditions surrounding most of these teas were oriented towards their use as medicine. Freshly gathered or home-dried herbs are usually clean and safe enough for a healthy person to consume without sterilizing them, but if you're trying to treat someone who's already sick and vulnerable, boiling your materials may be wise. Our modern culture and economy tend to radically simplify such traditions to make them sellable, and then even those less capitalistically inclined wind up repeating the same lines they've seen on the sides of chamomile tea boxes, and before you know it everyone's referring to using a single steeping method for a very broad range of very different plant bits.

I'd recommend experimenting to figure out what works best for any given herb, with the caveat that certain things like dried elderberries quite possibly should be boiled regardless. I'll nibble a few raw myself on occasion but sensitivities differ.