Lightning bolts on average are 8-C (1.6 gigajoules). I've seen higher values too, all the way from
2.4e-7 to 2.4e-6 megatons of TNT (
1.004e+9 J to 1.004e+10 J, which is high-end 9-A+ to just above baseline High 8-C).
But most of the time, ordinary humans don't take the full yield of the energy, either because they're on the ground which allows the electricity to be safely grounded and dissipated, or they're not usually directly struck and instead struck through a crapton of stuff (Like air, trees, some other material, etc.) by which point the electricity has already met tremendous amounts of resistance and lost most of its energy. So nah, it's not because of the heat that we don't scale electricity, it's because the humans are in specific conditions which causes the lightning bolt to lose most of its energy during the journey before impact.
This link explains everything well. Usually, they survive via sheer luck, or through good planning, but most of the times taking a bolt directly and head on is suicide.
Which is why in order to scale to the full yield, you have to be in mid-air (Where there's no solid ground to save your ass), the bolt must be in close proximity (Usually the case in fiction where several storm-based characters can transform into natural cloud-to-ground lightning, storm-ride as lightning and even shoot big-ass lightning bolts to one-shot entire cities and leave smoldering craters in their wake but at that point you might as well calc the city destruction instead), and/or your body must be bloodied and bruised so that the electricity meets little to no resistance and can pass on its full energy yield through your body (This one isn't particularly mandatory by any means but helps the case for scaling).