Speaking from experience, and as a female who also took sex out of my marriage, I will give you my honest reasons for doing so. I am 53 now so a similar age to your wife.
(1) My husband wasn’t passionate. Ever. There was never any play, kissing (he didn’t like it), teasing, hand-holding…just boring sex. I hadn’t been with anyone else to compare.
(2) There was a ten year age gap. He aged faster than I did. I found him old in the way he looked and the way he acted.
(3) He was very cold, emotionally. Never said he loved me to my face. Lacked emotional connection.
(4) He never seemed to get horny so not sure if he had a low sex drive. I did have suspicions he was gay but not sure.
(5) I got the ick as the years went by. We hadn’t had an active sex life and I became bored and frustrated. It was like a chore. I stopped it. He never asked again. I matured and changed. He didn’t look after himself.
(6) I knew I was living in a marriage devoid of passion, intimacy and sex but made the decision to live with it. We got on ok aside from this (although that ‘spark’ was never there). I was 35 years old the last time we had sex. It was rubbish. I got nothing out of it.
(7) At 45, I hit menopause. My main symptom, a few months after my periods abruptly stopped, was high sex drive. I was insatiable. However, I still could not go near my husband. I started to realise how I was feeling and how I was living. I suddenly realised that I wasn’t happy in the marriage. I definitely wasn’t attracted to him and had never had the heart to tell him. I wish I had.
(8) Another man came into my life (he is still in my life years later and, like you, lives in a sexless marriage and won’t do anything about it). Yes, we got involved. Big time. I have never experienced so much passion in my life. My eyes were opened. My sex drive with my husband had been tanked, not because I naturally had a low sex drive, but because I was with someone who didn’t bring out the best in me, someone I thought a lot of but wasn’t attracted to in a sexual way. He didn’t turn me on, basically. I ended my marriage immediately (once I got involved with this man) and my world was turned upside down. My sex drive was so crazy going through the first few years of menopause that I ended up with this man in my life - I know how very wrong it was. He is still in my life. We are very good friends now and we have had amazing times. Sadly for me, it’s her he loves, and insists he is coping in his sexless marriage. He isn’t. He gets ridiculously horny! He has cheated numerous times. Meanwhile, I have lost my marriage, home etc. and am still alone.
I am still very sexual and crave that closeness that intimacy and sex can bring. To find someone who you get on really well with in and out of the bedroom is amazing. I wish I had had that (I wish this man had given me that but I was stupid enough to fall for someone who was never going to be mine). Big mistake. But, it opened my eyes.
I regret not ending my marriage years earlier. It was very obvious I viewed him as a friend. I was lonely in my marriage. I wish I had acted earlier and said something. For both of our sakes.
My guess is that your wife isn’t attracted to you. Sorry. You need to have a real conversation with her. You are entitled to a happy, fulfilling life. Are you getting one? No. She will not change. Your high sex drive is not a curse - it’s a blessing to the right woman.