Because as anime shows, all Japanese people are connected to interdimensional reservoirs of fluids.

冷汗三斗
rei.kan.san.to

Literally: cold – sweat – three – to [a unit of measure]

Alternately: Breaking into a cold sweat due to extreme fear or embarrassment.

Notes: A 斗 is equal to ten 升, or in more familiar terms, 18 liters. 三斗 is thus 54L, or about 14.26 gallons – a comically vast amount of sweat.

The final character may also be pronounced voiced, as do. A synonymous compound replaces汗 with mere 水 (sui), “water.”

Gotta keep cool, see. Text says “sweat like a waterfall.”