Good point.Now to address your actual question. The most simple and concise answer is she has all the power.
The second you get with someone that actually wants to be with you and has genuine desire for you, she looses all her power and control over you.
She knows that your affections and attentions and ultimately your resources and money and support and provisioning and security etc will shift to the other person.
Right now she has you right where she wants you. She gets all the benefits of marriage and having a man around to buy new furniture sets every few years and that unclogs toilets and kills spiders and brings home $$$, but she doesn't have to do anything and doesn't have to put up with any of that icky sex stuff...... at least not with you.
When she says it's not important, she means it is not important to HER because she is not attracted to you and does not desire you.
However, she knows fully well that it is important to YOU and that if someone else was providing that to you, that you would soon be moving on with that person(s) and as the Eagles said in the song "Already Gone" "But let me tell you I got some news for you, and you'll soon find out it's true, and then you'll have to eat your lunch all by yourself."
So no, it's in her best interests to keep you captive in the dead marriage.... and I have to say she has played it masterfully. She dangled the carrot like a zen master. You have been on the string like the perfect puppet.
At the moment you value your money, furniture and 24/7 access to your children more than your own sexuality and well being. That may or may not change in time.
If you value your property, financial accounts and status quo more than your sexuality and happiness, you will remain in your sexless state.
The day that you value your own happiness and sexuality more than the new furniture, you will walk.
Many want a sure exit strategy. From having nothing to cling to at night, to having a chest full.
Classic honey/hunky branching.