Because many women in his age group have learned over the years to appreciate and give credit to someone who gives a sincere compliment.
Many women in your age group have neither the experience or confidence yet.
Also, when you are a young woman, compliments can come at you from all sides, and you might take them for granted. When you get past a certain age and are "off the market", It's a delight when a nice compliment comes your way.
Quick story from when I was in my forties.
A young man, Marshall, who was in his 20's had asked me how to do something at work, and I explained it to him. He asked me how I knew that. I (jokingly) said, "When your old like me, you know these things." His response was "Oh."
I turned to another 20 something co-worker, who was a great guy, and quite a charmer. He knew why. He immediately said "Oh Chai! You're not old, you're a beautiful woman in the prime of life."
I looked at Marshall and said "That's what you were supposed to say".
BTW, Marshall didn't have a girlfriend, I don't think he ever had one. Mitch on the other hand, oo la la.