I think most commonly: have minimal responsibilities to begin with (no kids, pets, business to run, etc.) If those aren't an issue, costs are pretty minimal, and it's mainly a matter of the long term career and retirement savings implications of not working for several months - those wouldn't stop you if you wanted to do it, they're just a judgement call.
I crossed North America four times in my twenties before I had a kid. I was a teacher, and rode west to east for two summers. Then I quite my job, and circled North America on a bike. It was the best choice I ever could have made.
I'd have a hard time riding across the country now just because my kid is still in school. But when he's out on his own, I'd like to do one more cross-country ride. I want to see how much has changed, but more importantly what has stayed the same.
77 days is quite a while (and assume a few more for final transport home), but I get 5 weeks of leave a year and can rollover 6 weeks from year to year, which, with weekends, would get me 79 days. This is not even a stellar job. Of course, being away from my wife (no kids yet) for 77 days might be a hard sell, and when we do have kids that'll be off the table for a while.
Then you schedule the event to coincide with holidays. In the US, we have (depending on your employer) 3 federal holidays from May through July (Memorial, Juneteenth, 4th of July). 4 for September through November but it doesn't line up well enough to get all 4 starting with 11 weeks off (Labor Day, Columbus, Veteran's, Thanksgiving). If you're willing to travel to get a better climate for an event, you have November-January which gives you Veteran's Day, Thanksgiving, Christmas, New Year's, and MLK.
So 3, 4, or 5 extra days on top of leave are feasible several times of the year (late spring/summer, fall, winter). That can be used to stretch your leave if you don't have enough to make it, or to give you a recovery week (or partial week).
Or, you get enough money and have no responsibilities and just quit working for 2-3 months.
Probably, but lower pay than most software engineers at this point in their career even excluding SV and major metros. It's a tradeoff. I could almost double my salary going across the street but I'd have to drop to 4 weeks of leave a year and lose the rollover.
EDIT:
But if I'm understanding the table you linked, for people with between 10 and 20 years of experience, 19% have > 24 days off and 43% have > 20 days off. So my base leave is above average but not extraordinary. The rollover may be, though.
I've had several coworkers take 1.5-2 months off at once, usually scheduled very deliberately to coincide with the completion of some major work effort (+ some margin), when work would normally be slow as the next effort is barely ramping up. Of course, part of doing this means effectively not taking leave for a year so you have enough to roll over or using smaller amounts of leave for several years to accrue enough rollover leave.
This is far more realistic, and likely more enjoyable. Nobody should be jumping from casual weekend rides to something like the tour divide or comparable. Do an overnight or weekend, than plan a multi-day, then do a longer distance trip to figure out if you actually like it, then pick something ambitious.
Don't know about the folks on that website, but a lot of folks who do these longer rides (i.e., one or more weeks) are retired and/or older and in a good financial position to where they can take the time off (also no young kids at home).
In particular, I wonder how people set up their health insurance while on these epic bike tours. It makes sense that costs are minimal, you can have savings, etc. but insurance is usually tied to employment around here..
If your income has gone to/near zero in the US, you're immediately (next month) eligible for Medicaid. That gives you emergency coverage and if you're healthy enough for a long bike tour that might be sufficient. Getting prescriptions filled out of state could be a sticking point though.
