We're talking about RAM. RAM usage is a direct function of how many apps you have open at the same time. For an iPad that number is a lot smaller than for a laptop.
I don't think that's the reason the iPad Pro performs well at video encoding (or else you'd be able to say the same thing for any Android tablet). It's a combination of dedicated hardware and a fast CPU.
What I'm saying is that just because the iPad Pro is good at video encoding it doesn't mean that it is capable of running a windowed OS with real multitasking. For that you need more RAM. The iPad is built for a specific workflow where you use one or two apps at a time, laptops are designed for a workflow that involves more multitasking.
What Apple appears to have done is transpose the hardware from iPad to Mac without thinking about the different requirements of each. There is a reason that RAM is not part of the package on laptop CPUs. This might work for casual users but people spending $$$ on a "pro" computer will wise up pretty fast.
I used to run a Windowed OS with real multitasking on a 33MHz 486 DX with 4MB of RAM! Sure, apps are a little more resource hungry these days, but there's nothing about multitasking or windowing that inherently requires tons of RAM. An iPad could for sure enable full multitasking without any problems. Apple has made millions of laptops that ran OS X perfectly fine with less than 6GB of RAM. It's just that iPad users aren't accustomed to having to manually manage which apps are open at any given time.
I have a 2019 Macbook Air with 8GB RAM and it runs OS X fine for dev work. RAM compression and fast SSD storage make a big difference. The newer Macbooks will have even faster SSDs.
I think the main reason Apple don't want to add lots of RAM to the base Macbook models is that it's the wrong compromise between battery life and performance for most users. They are probably betting on continuing gains in SSD performance obviating the need for additional RAM.
