Ever heard of Apple? They recently announced Apple Silicon which is basically a beefed up tablet/smartphone processor for notebooks.
The problem is, that most consumer PC run Windows. Windows uses the x86–64 instruction set. Common cellphone CPUs don’t run x86–64. Microsoft is working on an ARM based version of Windows, but at the moment it is only a niche product. So if you want to run Windows you are going to need an Intel or AMD processor.
These cellphone processors also use less power, since they have to do less. They are running an extremely simple OS with cut down software compared to a desktop.
Someone pointed out, that macOS isn’t a lightweight or simple OS. That is true, but the advantage for Apple is, that they can optimize the OS for their CPU, whereas Microsoft has to make Windows be able to run many different systems making it far more complex.
Someone also pointed out, that Microsoft has a consumer product (Surface Pro X) that runs Windows on ARM. It has actually been on the market since 2019, predating Apples switch to ARM by about a year. I have to admit, that I wasn’t aware of that at the time I wrote my answer. Be that as it may, Windows on ARM is still not available as a consumer retail product so in my eyes it stays a ‘niche’ product.