That might be the case with your vehicle, but on well designed vehicles like my Defender, the battery is well away from the heat under the passenger's seat. :D:D:D
Maybe there is a greater need to keep the battery out of the engine bay when the vehicle is expected to do a lot of hard work at low speed. I'm just guessing that the heat build up in the engine bay would be worse in that situation.
I suppose design always involves compromises and there are probably some advantages to having the battery in the engine bay.

