One of the reasons people bake sweet or white potatoes as a substitute for french fries is to significantly reduced the fats.
If you use an oil spritzer and spray a whole sheet pan of the potatoes with a Tablespoon of oil...
AND you only eat your (quarter sheet panfull) share, you will only be eating a quarter Tablespoonful rather than a quarter cup of oil when eating deepfried.
By eliminating the oil, as you witnessed, you have a hard time with seasonings AND you might as well be eating a baked potato.
The cooked oil and caramelizing of the potatoe are what contributes to the great taste. Can't get anything close to that without using at least a tiny amount of oil.