And it also depends on why the bags are heavy. If you have a good explanation that the airline believes and accepts, you may get waivers. If not, you may have to check the bag, if the weighing is being done while you check-in in person for your flight at the check-in desk where they have plenty of weighing stations. It may therefore be that it's less likely if you check in online (and go straight to the gate where they're less likely to have a bunch of weighing stations), but for some international flights it may impossible to check in online.

And I would guess that some people who want to ccarry-on heavier stuff may choose why airline to fly based on that. For example, you can fly between much of South America and the USA on either LATAM or AA, and with AA you might have higher prices sometimes but no weight limit on carry-on, while LATAM may have better prices sometimes but does have a limit on carry-on.