@fuuuuuk
To answer a question you didn't ask first (but I see asked earlier in the thread) why do people in our position get married to our partners if things aren't great...
For me I was basing my assessment of the situation on lack of experience and confidence, which I wrongly assumed would improve over the years as we grew together as a couple. Obviously it didn't. I also felt that perhaps my own focus on sex was misplaced in the grand scheme of things and that it'd not be an issue long term. It has been.
Then there is the clarity of 20/20 hindsight, coupled with the fact that your perspectvie on yourself, relationships and the wider world is very different in your 20s than it is in your 40s etc. it;s easy to be wise after the event. Then there's the concept that you could spend forever trying to find that perfect person that ticks every box, but if you find them, they might also be on the same quest and you might not tick all their boxes, so you accept that perfect doesn't exist and some compromise will be necessary. Again, based on naivety of youth.
To answer your main question - despite all the 'LTB' (easy as that) style advice often given, it's never that simple. There are usually multiple factors to consider.
Rest of the relationship
Level of love
Children and their needs
Financial position
Etc, etc.
In my case, I love my DW very much, likewise my children, our family life and shared hopes and dreams. To give up all that, walk away, break those hearts, cause all that upset, hurt and pain to people I love so dearly in the hope of maybe finding someone who can offer all that AND the sex life I look for is not a path I want to explore. To be clear, my DW doesn't see a problem, is perfectly happy and everything else is fine, so there's no impact on the kids etc. It's just me and my unfulfilled sex life.
If sex wasn't so important to me, I'd just dismiss it under 'oh well', but I struggle to do that, but am not prepared to leave my family in pursuit of the end of the rainbow. That doesn't make the problem go away however.
For the record, we do have regular, sex, it's just that it's not very good at all, for most of the reasons described by the OP. It's like being starving hungry and being fed food you don't enjoy - it keeps you alive, but you once enjoyed delicious food that made your mouth water....