Why aren't dual-SIM cell phones sold in the USA? Why doesn't iPhone has dual-SIM capability ?
In the USA, the four major carriers have service coverage just about everywhere in the entire nation - i.e., wherever people live and travel inside the country - either with their own native service on the tower in the area, or through relatively low-cost agreements with other carriers. The smaller carriers have agreements that usually achieve the same coverage.
And the rates (i.e., the plan costs for the customer) are the same no matter which state you are in - national service plans are the norm in the USA today. Indeed, as a result, many people no longer change their cell numbers when moving (temporarily or permanently) to another state, and the concept of a geographic "Area Code" has ceased to be useful for cellphone service. The carriers simply use the billing address to compute the taxes (state, local government, county, etc.)
In the USA, all calls and text messages are always done using the full ten digits of the directory number (called MSISDN) in the SIM, and the first three digits (i.e., the "Area Code") are just part of the number for the handset. Note that these 10 digit formats are called the North American Numbering Plan or NANP.
Thus, in the USA, there is no reason to switch from one carrier to another by using another SIM, i.e. to try to save money when traveling inside the USA.
Unlike what you might do, or have done until recently, in Europe when traveling often to another country. Because, for lower costs when roaming outside your carrier's native footprint in your home country (particularly if you do it often enough), it is best to get a "local" SIM from a carrier in that other country and use it in a second SIM slot. Using the second slot avoids the problems of replacing the single SIM constantly (pins wear out), misplacing one or the other SIM, etc.
In Europe, many carriers are government-owned or funded, and thus restricted to service only within their national boundaries - and roaming charges in other countries were fairly high until recently. So, dual-SIM phones are useful for frequent travelers.
Bottom line: there is no real need for a dual-SIM consumer phone in the USA, although M2M and IoT cellular devices are another story entirely! So the handset makers (like Apple with the iPhone) do not market Dual-SIM phones ... there is no reason for the additional physical space for a second slot, or the code and firmware to support the second SIM, etc.
Syed had given a great answer. I'll add a further perspective that may or may not be accurate.
When mobile phones first came on the scene they were sold through exclusive contracts by carriers. The carriers had no incentive to implement dual sim roam. This is pre iPhone and Android, so the phone manufacturers had little power to add features that ONLY benefited the user and would undermine the revenue of the carrier that had sold the phone.
So we all got used to single sim phones and the use case has continued to be marginal. But with android and iPhone now independent of the carriers those vendors can implement dual sim to sell a few more products.
Where did you get this idea from? Just because the majority of everyday people don't use a dual sim card cell phone, or need one, doesn't mean they're not sold in The USA. Many USA retailers, both online and brick and mortar, as well as Telcos (telephone companies), have been selling them to consumers and business travellers for many years now, they're just not in much demand for them, hence they're not normally displayed. You have to ask for them and they'll bring them out for you to see.
Most of the name brand companies make several different versions of them. Samsung, Sony, LG, Nokia, Motorola, Huawei, Kyocera, Xioami, HomTom, DooGee, Philips, Lenovo, etc., etc., and etc.
In other countries like Europe, and in the African continent or Asia, it's very common to see dual or triple sim card cell phones being used by the average person. If you look close enough you might even find them using quad sim card cell phones to take advantage of the various calling rates between carriers, especially when travelling.
You can buy a dual-SIM cell phone in the USA - here, for example: International SIM Cards; but the reason why they're not more popular is because until very recently, most phones were sold by a phone company. The phone manufacturers sold their stock to the phone companies, who then leased it to you as part of your contract. You used the phone, and they charged you for it, therefore making their money back on the cost of the phone. But if it had been a dual-SIM phone, they would potentially have made less money because you could have used another company's SIM. And the phone would have to have been sold unlocked, which might have meant that you didn't use their service at all. So it has been in everyone's best interests to keep phones to one SIM only, since they're so easy to switch out when you need to.