In answer to your question, probably sitting in a lonely marriage, or got wise!
"Define Nice"
'Nice' isn't a trait that promotes desire, excitement, nor is it a trait that promotes loyalty, my guess is that most genuine 'nice' men are avid providers, emotional rocks and give or at least try to give 100% in relationships to make their partners happy, but they won't be perfect and won't always get it right. The problem is that the excitement wanes, the nice guys are reliable, dependable and grounded, true family men, but they aren't dangerous, unpredictable and exciting...it's almost impossible to be both, once they realise this and figure out that the reason they are still married has nothing to do with being desired or being wanted emotionally, the realisation hits hard, they are merely lifestyle enablers or moss covered rocks, at this point they probably eventually stop being nice guys, or decide that relationships are all the same so why bother, which is why you can't find any, sad right!
I may be speaking out of turn but most of my female friends will tell me this, (and i find it hard to disbelieve based on real life experiences) we don't look for nice guys, nice guys aren't exciting, we look for projects, men who are exciting, dangerous, unpredictable, independent, full of swag and over time we try to mould them into what we want them to be, they then either refuse to change and it goes nowhere or we stick with them and find it hard to manage the relationship due to us both pushing in opposite directions, or we finally start to mould them into what we think we want them to be, at this point they aren't that same person that attracted us in the first place they are now a nice guy that was once exciting, unpredictable, dangerous and full of swag...and we start to get bored...so we seek excitement elsewhere again...
I can tell my wife she's sexy, beautiful and the best mum in the world to my kids everyday and it will mean very little, a good looking stranger however can say the same thing once and it would be life changing.
I guess the one thing i've learn't over the last 50 years is that being nice gets you nowhere, although i can't imagine not being that way, and you're just playing for time before someone better comes along, with those easy picking compliments and blows your world up. Life hey!
This is not an assassination attempt of either sex, this is unfortunately how desire and attraction is programmed into humans, hormones and their imbalances control our desires, you aren't wrong to desire what turns you on even if you know they are players, you don't control that, nature does. But don't be surprised when nice men become impossible to find, especially when they have 50 years of experience under their belt.