I don't know. Maybe it's the part where you can't swim in lava and will just fall deep through it like air. Maybe it's the part where the lava crits me for bonus fall damage after I slip in. Maybe it's the fact that unless you're already attached to a grapple and just accidentally brush the lava, if you touch the lava it might as well be an insta-kill because you're gonna die guaranteed anyway.
Or maybe it's just all the 1x1 holes that are fifty blocks deep hidden in the middle of otherwise innocuous floors that I like to sprint across while mining gems and coal.