Why should I eat excessive calories and gain weight?

Male, age 65, 127 lbs, 5' 5", waist 32, hips 34, forearm 12.5, wrist 7 = 8.3% fat.  Tennis 7 hrs/week.  Weight exercise at the gym 2-3 hours/week.   By actual measurement, weight is gained if calories/day exceed 1500.  The website says I need to eat 2400 to 2700 calories/day.   This would be insane because my belly would probably go up to 40 inches. I would not be able to play tennis.   Why do you want me to be fat?  I'm muscular and feel great.  If it ain't broke, why fix it?   Why is your website giving out wrong answers?  I challenge you.  What are your credentials?