If you weigh 100kg, drinking 90*100g=9kg of water produces a 50% chance of fatality. The definition of LD50 requires the dose to be given “all at once”, and quite frankly, you would not be able to drink 9 liters of water all at once. LD50 becomes a lot less meaningful for anything where you would need an extreme concentration of the substance–e.g. THC is difficult to acquire in concentrations compatible with fatal overdose–or where consuming it at such quantities is simply infeasible.
People often say “consumed rapidly” but that phrasing doesn’t really solve this problem with LD50 as a measure. Basically LD50 is meaningful near the bottom of this chart, less so near the top of this chart.
I’ll note that another problem with LD50 is that it doesn’t take into account serious harms that can occur with lower dosages. Drinking any amount of gasoline is likely to lead to serious brain damage, for example.