[pdx]

If you re-read my comment, you'll note that I only said that 2.4GHZ was in the 1-20 GHz band that is easily absorbed by water.

Engineers juggle multiple constraints, cost, size, power, and efficacy, and 2.4 GHZ choice was the result of the size of the magnetron that would be cost effective and physically fit into the oven. But it did, of course, have to be within the range of frequencies best absorbed by water, which 2.4GHz is, and which is why it's difficult to get bluetooth through your body.