Yea, that does suck. They are clear in their documentation when it says "Apple Watch Ultra, Apple Watch Series 8, and Apple Watch SE require an iPhone 8 or later with iOS 16 or later."
But it's buried in a giant page of fine print that you'd have to know to go looking for. Of course if you know to look for it, then you probably don't need the information.
Anyways, I agree that it sucks to have such a limit to JUST an iPhone. It would be nice if it worked with an iPad as well (which is really you're only other option). But the watch, watch app and others make implicit assumptions. For example your iPhone app, which is separate from the iPad app, also has the support for the watch built in (aka the watch specific apps). The iPad apps would currently not have this, because that isn't how it works. Could that change? Sure. But I would guess there are a lot of other implicit assumptions made that we're not aware of.
Lets be honest... Apple would be happy to sell you a watch AND an iPad. You'd be very likely to eventually get the Phone as well. But realistically that's not the direction most people go. They get a Phone first, then maybe they get the watch or iPad.