I think the principal issue is the kind of use people want from the bikes. To answer the OP's question, I think that manufacturers are recycling road bike engines and basic chassis set ups and calling them "Adventure" bikes.
What they are, in fact, is "Touring" bikes.
At the other end of the spectrum, what people are looking for is the old fashioned "Trail bike". Not a high maintenance "Enduro Bike".
The problem is, at the lighter end, no one is doing "Trail Bikes" with any seriousness. Yeah, Yamaha still do the TTR250 with a reliable low maintenance engine, and the WR250R Supertrail...but like someone else said, riding at freeway speeds on a 250 is over zealous at best.
Many have asked for a WR450 Supertrail. That or a better suspended version of the little Tenere would be the go. Maybe even a 500 version of the TTR. They used to have one, surely the engine is still used in something?
I built one of my DRs to fill exactly that role. MX suspension, lazy 650 engine. It fits the bill so perfectly. Yeah, it could be lighter. But, to be honest, I'm usually only thinking that when it's laying on top of me.
I'm thinking different thoughts when the BMW is on top of me. I carry a survival knife for a reason. I'll probably have to hack my own leg off one day. Fat pig of a thing.