Why do doctors consider a gfr over 60 normal when 60 is considered stage 2 chronic kidney disease? seems like a fine line!

Not a firm number: Normal GFR in younger persons is 90-120 ml/min. The number is an estimate and depends on a bunch of factors including age, sex, weight, race, height and creatinine measurement. So the line is not fine but really fuzzy.

It's mild: Gfr over 60 is a mild loss of kidney function. Blood tests may be mildly elevated, or still normal in an older person. No symptoms or complications expected until later stages. Can do very well with prevention of more kidney damage by controlling hypertension, diabetes, avoiding kidney toxins, etc.

See below: There is increased risk of adverse renal outcomes for egfr<60 even without albuminuria. On the other hand there can be overdiagnosis of CKD in the elderly and e GFR >60 should maybe be termed risk factor or pre disease.