[Samsung Galaxy A51 Guides, News, & Discussion] sjevtic:
That's basically true. Modern U-devices (USA market intent) have locked bootloaders that do not readily allow unlocking, which is a problem even for rooting since the most common way to do that these days is to install a Magisk-patched boot.img.

It's even more complicated than that. Google/Android and Samsung are definitely part of the movement, but in the USA, carriers also play a bigger role in device distribution than many other parts of the world. Since carriers are responsible for so many device sales here and often have a lot of skin in the game in the form of creative financing of these devices, it's easy to understand why they would want--and device makers are happy to oblige requests for--heavily locked down devices (i.e., non-unlockable bootloader).

Moreover, high assurance functionality is a selling point for in excess of 99% of a device's user base. While PCs continue to be readily infected with malware, locked bootloader devices with a full chain of trust are quite a bit more difficult to compromise. The average consumer, who has never once though about rooting a phone, takes comfort in knowing that his or her sensitive data is safe at all times, even if the device is lost or stolen or if he/she isn't the best at defending the device against malware. Enterprises want to ensure that mobile devices aren't the weak point in their security that allow sensitive intellectual property to escape.

So, what we are discussing here is likely not even a 1% problem. It's more like a 0.1% problem. I work in a technical field, and despite having a technical group of associates, I literally do not know anyone other than myself that roots his or her phone.

That would definitely be the easiest way. I have been using international Exynos-based devices for years now quite satisfactorily. A lot of times they can be had a bit cheaper too.

One thing you have to watch out for when going that way is carrier policies on IMS, which is the technology that powers VoLTE and VoWiFi which are rapidly becoming must-have features. AT&T only allows whitelisted IMEIs to access IMS on their network which basically boils down to iPhones, Pixels, and branded phones that they sell. Without IMS access, you'll drop back to 3G for voice and data, and as of right now, AT&T is planning to discontinue 3G voice/data as of January 2022, soon rendering an international Samsung device completely useless on their network.

It seems that the enthusiast community has made some progress in unlocking these U-devices though. @elliwigy runs SamPWND, a business that specializes in remotely unlocking them. I haven't used his service yet personally since I avoid these U-devices, but the response around the XDA community generally seems positive; you can search a bit for yourself. It's understandable that the service isn't free, both because of the hard costs involved (Samsung-signed tokens and remote access infrastructure) as well as the considerable research and development required. Do you know how to deliver a patch to the steady image on a locked bootloader device? Do you know what to change in the steady image to change the lock state of the bootloader? Well, that makes two of us.