This is quite true. I had nothing but disdain for Flash back in the early noughties because, almost universally, everything I encountered that was built with it was terrible, especially those CPU core swallowing ads.
Nowadays of course we have CPU core swallowing and battery draining JavaScript ads, along with massive JavaScript bloat in general. For the heck of it I just went and had a look at one of my own pages, which runs an HTML5/JS/CSS game, and includes Adsense. It downloaded 633KB of JavaScript. By today's woeful standards that's not even particularly bad. To break it down a bit though, 52KB of it is scripts I use directly, of which 47KB is required for the game. The other ~5KB is FontAwesome, which I use for icons on the menu buttons. The other 581K? Well, Adsense itself is also fairly small (about 35K for two scripts), which leaves the rest - the vast majority - which is loaded by the ads themselves.
Like you, I've come to realise that it wasn't the tool/platform, but the people building for it. Clearly the best developers are not building ads. It feels like there should be some scope for disruption here.