Evenly distributing 98 markings is hard. Evenly distributing 64 is easy -- just halve the range six times. That's why the distance from freezing water to body temperature is 64 Fahrenheit degrees.
Freezing point of water: 32°F
Average human body temperature (oral): 98.2°F
So that's 66.2°F, not 64°F.
Those two temperatures were the original reference points for the Fahrenheit scale, and body temperature was defined as 96°.
The current definition is relative to the melting point of ice (32°F), and the boiling point of water at standard atmospheric pressure (212°F, which is exactly 180°F higher than the freezing point). Except now body temperature is 98.2°F instead of 96°.
If you want a thermometer calibrated in Fahrenheit, you have exactly the same problem as you do with Celsius. You just have to put 178 markings between the two reference points, rather than 98.
Which makes no difference because you're eyeballing it to start with. You can just as easily divide the 0-100 range in half a bunch of times. Your thermometer is not achieving <1 degree accuracy.
Of course if you have a tool to accurately measure halves, then you can also measure any other division you care to use.
