Loose hub bearings may be cheaper to mass produce, but why wouldn't sealed hub bearings be one of the first upgrades to a bike? It seems that most of us choose loose bearings. I'm thinking of putting in a set of sealed bearings front and back and be done with it.
There are some misconceptions here.
Loose bearing hubs, which I think are more properly called cup and cone hubs, can and do in fact have seals. This 2013 blog post describes modern Shimano hubs, which are still (!) cup and cone, as having both contact seals and non-contact labyrinth seals. Historically, Campagnolo and Shimano have both used cup and cone hubs. Campagnolo transitioned to cartridge bearings on many of its wheels in, I think, the early 2010s.
The proper term for the other arrangement is actually cartridge bearing. The cartridge comprises the inner and outer races, the bearings and a bearing retainer, plus whatever seals or shields. As I mentioned in comments, you can buy cartridge bearings with less-protective metal shields, although I don't think that arrangement is OEM spec on many (or maybe any) bicycle bearings. Because the seals exert some drag, if you were trying to set the hour record, you could even take the seals out of your cartridge bearings - since the hour record is set on an indoor track, which is relatively clean, this might not be an absurd thing to do for someone who actually has a shot at the record. Regardless, the point is that seals are not necessarily intrinsic to cartridge bearings and cartridge bearings alone.
In any case, cartridge bearings are in fact commonplace on modern bicycles. This blog post by Wheel Fanatyk thinks that Phil Wood, Maxicar, and possibly Chris King were among the pioneers of cartridge bearings in bicycle hubs (Phil) and headsets (CK), possibly starting around the 1970s. Since then, cartridge bearings have become standard in industrial applications. There are a large number of standardized bearing sizes, many of which are applicable in bicycles.
There's a classic make or buy decision at play here. Depending on your requirements and your capabilities, you might be able to make everything yourself, or you might be better off buying some parts from someone else. Given the wide availability of cartridge bearings of different sizes and quality, I think most cycling companies have chosen to go and buy their bearings from someone else, e.g. Enduro or SKF bearings. This could allow companies to focus on other aspects of their product, be that hubs, bottom brackets, or something else with bearings.
There are some exceptions. Chris King, an eponymously named small boutique manufacturer, uses cartridges but they make their own bearings in-house. I recall a podcast where King described how he scavenged a large bin of barely used medical bearings, cleaned them out, and put them in headsets. Shimano is a very large manufacturer of many types of cycling equipment. As I mentioned, they still use cup and cone bearings in their hubs. They certainly have the scale to keep producing high-quality cup and cone bearings, and I wonder if their institutional conservatism (or at least, that's how I perceive them) may have pushed them towards deciding to make, rather than buy. Campagnolo seems to have recently chosen to buy at least some of their wheel bearings, despite long prior experience with making very good cup and cone bearings. Campy is a much smaller operation than Shimano. This is pure speculation, but their smaller scale might have pushed them in the opposite direction from Shimano.
Finally, just to clarify, you can't replace cup and cone bearings with cartridges, or vice versa. You would have to replace the whole hub or bottom bracket.
Cone, ball and cup aka traditional wheel bearings are the simplest and cheapest expression of wheel bearings. Simple to manufacture, easy to maintain and repair with the simplest tools and a handful of standard parts that have been around for more than a century in the remotest locations. And the wheel will even turn after emergency repair when, say, one bearing ball is missing, while a toast sealed bearing means a break-down.
Sealed bearings require high precision manufacturing, logistics & stock-keeping, special tools and trained mechanics.