Travel insurance is so cheap you'd be insane not to.
I'm picking on one example, but all of you saying that travel insurance is cheap have probably never tried to get medical insurance for someone with a chronic-but-harmless illness - that is, one which I'm told if treated has no effect on life expectency because the treatment basically reduces the risk of being killed by the illness sufficiently that the beneficial side-effects of the treatment outweigh the residual risk. In other words: the main difference between them and a member of the general population is simply the diagnosis... and a fraction of the general population will have the same illness, undiagnosed, with the elevated risk... plus the diagnosed people are routinely checked for a range of other conditions like diabetes, so insurers will usually know if they've got that.
But simply by being diagnosed and treated, the insurance gets loaded - I'm paying about 5 times as much as if I didn't have a chronic illness. Unless the medics are blowing smoke up my arse about the benefits of treatment, I think this is a market effect rather than actuarial adjustment. I have to buy from a smaller market of insurers who seem fairly close on price and even those won't give a representative price until you go through screening - despite a range of starting prices, the wildly different loadings seem to put most within £30 of each other, usually within £5. Some of the large providers are known for flat-out declining certain conditions so you can't ask them else you have to answer the "have you ever been refused insurance?" type question with "Yes" and that shrinks your potential market even further. It seems like a market for lemons.
And that small market is before you start excluding insurers with onerous restrictions on physical activities like cycling.
All this takes farking hours and often phone calls, too. And that's if you know it's coming and keep your own medical notes (I do... I've been ill a long time). Some of the medical specialist insurers even ask if you've had a common cold in the last year!
I still wouldn't travel without insurance but I can understand why people take the risk.