Do you drive a car without shocks and springs or struts ?

Same reason for FS bikes. They hold traction better in all situations.

Some places a FS is overkill as conditions aren’t severe enough to warrant the trade off of increased weight and percieved loss of power to the rear suspension. I just went to a new HT as for the majority of my trail rides an HT works really well. Introduce rocks, steep descents and drops and the situation changes and a FS is a better choice.