I've heard it's because baby can't regulate their own temperature yet...so if you heat them up too much they can't cool down. But I'm not sure, so maybe research it?
I was told you can take hot baths and showers just not hot tubs . Reason being a hot tub is at a steady temperature as a shower and bath can and will get cooler as time goes on especially a bath .so it will not over heat your body or rise you heart rate . However don't make your bath super hot to begin with .
It brings up your heart rate which is bad for baby and can "cook" the baby if it's too hot, because it brings your temp up. As long as your skin doesn't then red or your face doesn't feel hot it should be fine