There are two possible explanations which occur to me here. Unfortunately neither offers an obvious solution.
The first is simply that the RAM management is not as well tuned as it could be. The manufacturer does make some choices here, and some don't do as well as others. I have a Galaxy Note 10.1 (2014 edition) tablet and my previous phone was an HTC One (m7). The HTC had 2GB RAM while the Note had 3GB and was used more lightly than the phone, but it was always the tablet that did more app reloads and generally multitasked less well. Samsung do load more stuff into the ROM, but even so given the extra GB I always suspected their tuning was inferior. (As an aside, there have been much clearer-cut examples, such as the OnePlus software release for one device which always kept about half of the RAM free, with the result that it behaved like a phone with half as much RAM as it really had. The point is that the choices the manufacturer makes with their software can make a difference).
Unfortunately unless you are willing to root the phone there is nothing you can do here. If you do root it it's possible that there is a custom kernel out there that will manage this better, if this is the problem in the first place. But no guarantees.
The second is what's called a "memory leak". This is simply a bug, where something allocates memory but fails to deallocate it, with the result that RAM usage grows continually until something fails. This is a class of problem very familiar to C++ programmers, as C++ does not perform automatic garbage collection, but can occur in other languages too. If this is the cause then it could be a problem in one of your frequently-used apps (in which cases uninstalling or disabling it and using an alternative would fix it, if you knew what app it was), or it could be a problem with part of the OS (the manufacturer always has to adapt Android to their own hardware, and includes their own system apps and processes, so it's also possible for them to introduce bugs that aren't in the based Android version). In the latter case there will be nothing you can do unless you can root and find a custom ROM that doesn't have the problem. I was hoping I could find a system monitor on my phone that would tell me how much RAM each app was using, since something like that might enable you to identify the culprit (or might not: browsers naturally grow as you open tabs, so it would be hard to tell), but none of the ones I have seems to have that capability (at least not on my current unrooted phone).
I don't think this is a generic android feature, as I've never observed such behaviour myself. My current phone is on Android 11, so probably tells you nothing about your device, but even back when I was running Android 4 I never saw this type of thing, and would usually go months between reboots. But I never owned an S3 so don't know whether this was a common feature of that model.
