If each flight leg is a different price, how can the website show you the total until you select both (or all) legs?

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.