Fear. For sure.
But there's other things to.
5 or 10 years out, you hate your job. One reason for this is you don't think you have choices. I love my job but I still hate it. Standing on the edge of never working again, looking at my huge pile of assets, I know I'm going to look at a savings rate of over 50k/year and think to myself, that was actually fun saving that much money, and it's not likely I'll do that ever again.
Saving has become my favorite hobby. And the best way to pursue that is to keep earning the most I can, which I think is my job right now.
MMM said it several times, as well as ERE and others, it's about what is "enough."
You have this lifestyle you know you can live with, and you have this pile of cash you need to do that. But once you've gotten to the pile of cash, and it wasn't that hard to get it, you think, what if I just add one more thing to the lifestyle?
If working one more year lets you own your own plane and live on a house with an airstrip, that might be worth it. Maybe a relative just got in to college, and you think, I could cover that whole cost if I work just one more year. That's a gift to a loved one that will help them every day of their life. Maybe they'd be better off managing that expense on their own like you, but maybe there's no harm working that extra little bit.
If FIRE is about having the option to retire, then you've made it. To me OMY is about using that freedom and choosing to go ahead and still work.
The other thing, as I've gotten closer to FIRE, is that work isn't nearly as bad as it used to be. For one thing, I just don't do things that I don't want to do. They can F'ing fire me, I've got enough saved to live for decades. I come in late, I leave early if I want to. All my work gets done, I don't put up with any bullshit. A frugal lifestyle has already granted me many of the benefits of being retired. Knowing I'm almost done, I don't stress about "developing my career" the way some of these others do. I'm not going to do this for another 30 years, no matter what.
If it wasn't for having to be here so regularly I'd probably never quit. But I want to visit folks and be able to stay for weeks, instead of barely even two full days. I want to vacation in a place for long enough for it to be hard to say goodbye to the people I meet there. I'm 5 years away from even 3 weeks off per year, so FIRE is my way of buying extra vacation.
The why of OMY is the exact same why of why not now?
What keeps me working even now, until I hit my FIRE goal, is what if I can't really find that much part time work? What if the cultural stigma attached to retiring so young makes professional work impossible to get? What if zombies? The whole FIRE number 4% SWR thing is ultimately based on "maths" and hope, neither of which I'm inclined to trust. Is it fear? Yes. Or it's just being responsible. It's all well and good to have enough to live the rest of your life without being a burden on anyone. But if you fall short by 15 or 20 years, or have to put in 5 years of work after 10 years off that could've been avoided by OMY, then damn, what was the right call?
I'm not going to live off of 4%. 4% +other income will cover my expenses. Some on here would call me crazy, but I do have faith in myself. If I do OMY, it's going to be because work isn't so bad, the markets didn't do that well for the next 28 months or so, or because of a woman.
Two more years past my expected retire date, I'd save enough to completely pay off my house, eliminating my last debt and reducing my yearly expenses to under 10k/yr. It's tempting. It also would likely mean I'll die with 14 million dollars in the bank, which is just stupid.
I'd rather have to find a decent rock and a bridge when I'm 85 then for my heirs to have to deal with tax attorneys.