FWIW, at least as of today, American Airlines' website attempts to show you round trip prices.

When choosing your outbound leg(s), they show a price inclusive of the cheapest return journey on the day you selected to return using the class of service on your outbound leg. So, there's all sorts of ways for it to be incorrect - maybe you want a different class of service, maybe the cheapest return has a stop but you'd like the direct, etc. - but it's still really useful for figuring out the best options for your flights.

Kayak.com does it... it's very much a UX choice of whether to show combinations of flights at a given "level" (economy/main/1st class) or instead dedicate the space to showing the prices at all levels, and only show a flight at a time.

They usually show you a minimum, then have you select each leg, with the price for that leg fully displayed.

That doesn't mean it's not opaque and complicated.

The context is making pricing more opaque than it needs to be in order to earn more money.

I don’t understand how it could be made simpler, unless you want every flight to cost the same, which is stupid. Hence the complaint does not make sense.

"We can't tell you the exact price because you haven't told us what you want to buy yet" isn't opaque or complicated.

Kind of? The way flights are sold and priced is opaque and complicated IMO. That doesn't make it bad necessarily: maybe there's no good alternative, I don't know. But it really is opaque and it really is complicated. I think my opinion is at least somewhat educated, having done some crude flight price modeling for work in the past.