I'd bet that the reason web/mobile dev pays much better than embedded is that there are many more firms doing it. Embedded means that your company is producing a hardware electronics product, which is requires a specialized core competency that relatively few firms do. By contrast, web & mobile dev just means that your employer has some information they need to deliver to users, which describes basically all of the millions of businesses in the world. More potential employers = higher demand = higher wages, even if the supply of webdevs is bigger and becoming one requires less training.

(Personally, I found the low-level hardware/OS/algorithms courses fascinating in my CS degree, but went into webdev because it paid more and gave me more career options, and then some data science because the combination of UI skills + data wrangling skills means I can actually build useful things on my own. No regrets on my career path, but if I get lucky and cash out on a startup, I think I'd love to do embedded & hardware stuff.)