Bruce King has a sad post about a sow losing some piglets while farrowing outdoors. The weather has been warm (80s) lately, but it doesn't take much to kill a little pig. Besides getting crushed, piglets die of exposure, infection and other causes.
In general, people farrow indoors because it greatly reduces mortality and morbidity.
Farrowing outdoors can still be optimal economically though, because some consumers will pay more for pigs farrowed outdoors. Even if consumers don't pay more, it can pay to farrow outdoors because the low cost of facilities balances out the extra deaths - explaining why outdoor farrowing is still common in warm places like Missouri.
I've talked with people who farrow pigs and sell them to a large "niche pork" producer. As one guy said, he generally farrows 5 to 9 pigs per sow per turn, depending on the weather. If the general consumer ever makes the connection that "naturally raised" = unnecessarily dead piglets, we may see sentiment move against primitive pig keeping systems. At a minimum, if animal lovers knew the facts, they'd probably be in favor of farrowing crates.
Video on Sow Housing Options and Tradeoffs
Most people don't know about pig production, so they don't understand that there's many ways to keep them indoors.
For example, these Mangalitsa pigs farrow in pens. There doesn't seem to be a heat source for the pigs, but there's a rail to help prevent crushing. It doesn't look easy to clean though, so there could be various diseases in the building itself, continually infecting the piglets from turn to turn.
These F1 Mangalitsa sows farrow in crates. They've got a heat source in the corner for the piglets, explaining why they crowd there. Everything is set up for easy cleaning, like a hospital. This facility is better than the first, in terms of reducing mortality and morbidity - but it looks the ugliest (like a hospital) and bothers some consumers. I would want to be born on that farm, if I had a choice.
In general, getting the farrowing right starts will controlled breeding. A lot of small farmers just have their boars running with their sows. The boar breeds the sows when they come in heat. The farmer never knows exactly when that sow will farrow, because he doesn't know when she came in heat. When sows farrow in the wrong place (easy to happen if you don't know when she'll farrow), the piglets die or suffer, as explained in this video.
As soon as you've got farrowing facilities, which are expensive, you want to avoid putting sows in them unless you know they'll farrow soon. Hence the need to control the breeding. One way is the "heat check" the sows regularly (e.g. once or twice a day), to see if they are in heat. But that sort of routine isn't very compatible with other things. You might as well just run pigs all the time, if you've got the facilities and skilled labor to do it.
This explains why people specialize, and why we only see a few different ways of keeping pigs in America. E.g. you'll have a small number of people running a large number of pigs (like these Iberico) in a very focused, modern way, or a smallholder running a relatively small number of pigs, chickens, turkeys, cattle, etc. It doesn't pay to be in the middle. E.g. you probably aren't going to have a small 4-crate farrowing house, next to the rabbits, turkeys and chickens. You might have 4 pens though.
Making it even more extreme: you aren't likely to see someone with a small number of sows kept in a gestation crates (small metal cages to hold pregnant sows during their pregnancy) - you either won't see them at all, or you'll see a lot of them (e.g. 200+). Over time, the minimal numbers that are optimal keep going up and up, as things get more efficient.
I've heard restaurants are the same: a lone chef can feed a few. One chef and an assistant can probably do meals for 15 people. Any more and he needs a lot more help. So you've got various modes that are optimal - and nothing in between.
When you do things small-scale, you can't be excellent at all of them, but there are advantages to it. If you are a smallholder and the pig business tanks, you get out of the pig business until it recovers. If you specialize in pigs, you are stuck.