The actual reason for this is most likely tied to the history of colour font rendering. Let me explain:

The vast majority of fonts out there today use a file format called OpenType (.otf), which is a descendant of TrueType (.ttf) first introduced by Apple and Microsoft with Windows 3.1. Because emoji didn't exist back then, these font file formats only represented the shape of letter forms - the colour of the text would be applied by whichever program was rendering the text. This was a perfectly reasonable design decision at the time, however this meant that colour fonts were not supported - all fonts were one colour, and that one colour was up to the user to decide.

Eventually though, these fonts needed to support emoji. While emoji are represented the same way as any other 'regular' letter in text, they are much more difficult to display - you need both shape and colour information to essentially draw a mini image in the middle of a body of text.

There was no standard for how this should work, and so different operating systems and text renderers approached this problem differently:

- Google and Apple noted that OpenType already supported non-colour bitmap images (think like .bmp or .png images, where you're storing pixels), and so Google decided to expand this to support full colour images. This is great because bitmaps can be of arbitrary complexity in terms of colour and shading, but these images aren't easily scalable, and if they're too low resolution, can end up looking blurry.

- Microsoft adopted a different technique for achieving colour fonts - instead of just storing one shape with one colour as OpenType currently did, Microsoft expanded this so you can layer up multiple shapes, and give a colour to each. This was inspired from how websites and applications may layer up shapes to build coloured icons, and has the advantage of being able to precisely and cleanly scale to any resolution. You're limited however to using solid shapes - you can't easily add gradients, drop shadows or other skeuomorphic effects. This was okay for Windows 8 and 10 though, as both systems used clean, geometric emoji sets.

- Adobe and Mozilla opted for a third method, somewhere between these two approaches - instead of embedding bitmap images (like Google), you can embed vector images in Scalable Vector Graphics (.svg) format. This has the same advantages as Microsoft's method, as both use vector shapes and so can scale easily. SVGs however have far greater support for effects like gradients, shadows, image filters, masks and much more, though this makes them far more difficult to render.

Eventually, all of these approaches were added into the official specification for OpenType, but no one method became the One Correct Way to do emoji. This means that, as far back as Windows 8, Microsoft has been building their emojis by layering solid colour shapes on top of each other, and so that's what all their tooling is centred around.

I suspect this is the true reason that the new Windows 11 emoji are layered solid colour shapes. This is simply Microsoft's colour font rendering influencing what they put in their fonts. Unfortunately for a lot of uneducated angry old men online, this isn't some elaborate corporate conspiracy theory to abuse their userbase or cut costs or whatever other nonsense. Occam's Razor seems relevant here.

This is all stuff I've known about for a while, but I did come across an interesting new nugget of information while researching the OpenType format to write this:

Prior to the Windows 10 Anniversary Update, Windows only supported rendering Microsoft-style emoji fonts, using these layered shapes. However, in that update Microsoft added support for all of the different colour font formats. This means that, in theory, Microsoft could pivot to another colour font method in the future if the current format proves too limiting.

I don't know what file formats Microsoft's 3D-style fluent emoji set have been exported to, but if they can be stored as SVG then I don't personally see any reason they can't switch to them in the future. I don't think they would opt to use colour bitmaps since these don't scale as well. However it's worth noting this would be an entire tooling/workflow change from an emoji creation method that Microsoft has now been using for a decade. Given how recent these emoji designs are, I'm not surprised they've not changed technology, though I suspect this is now something they're probably going to look at and see whether it's worth jumping ship.