I mean, maybe it's just me but I've had good luck with mine. The trick is you have to fill it with really hot water in the morning, like almost too hot to touch, then let it sit in the sun all day. If you just use lukewarm water to start, it never gets warm enough.
My buddy tried that exact thing last summer. He left his full bottle on the patio table all afternoon, came back to water that was basically soup. He said it was so hot it almost melted the plastic liner inside his bag. Kind of proved your point, I guess, just a little too well.