Dragon ball Z plot mistakes ?

I mean theyr body can generate even twice the power that the Earth can produce so my question is why the creators made the characters fear lava?"Lava is molten rock expelled by a volcano during eruption and it is a liquid at temperatures from 700 �C to 1,200 �C (1,300 �F to 2,200 �F)".Even at the beginning of DBZ in the saiyan saga in Episode 14- Princess Snake, Goku is taking a bath in a hot spring at 300 degrees and he is just fine like normal bath.So they(Goku,Gohan,characters) get stronger and stronger during the series and you would think that their body is durable enough to withstand the temperature of lava.Kind of confusing.