Embedded / firmware takes a lot more tedious work, often requiring more people and time and that's just for a single purpose device. Whereas a few Web developers can create a web service that runs an entire profitable company with tens of thousands or millions of users. Of course that's the generalization but I'd guess it's a 1:10 or at least a 1:5 ratio on the income/profit vs development time/effort required between web dev and embedded.

I strongly believe that projects like Nerves can help lower that ratio and make a lot of IoT type projects follow economics more similar to web development. Sure a RPi costs $35 per unit, but lets say you're making a veterinary monitoring system which costs $100k for the software it fits into the budget much easier if a couple of devs can implement it in weeks versus the months and/or large teams it used to take. There's a lot to be said for the long-tail of economics in areas where leveraging more productive tooling can increase productivity and profitability.

Unfortunately that only generally applies to roughly pre-canned hardware and commodity sensors, etc. You won't get that effect when building say a new Camera Sensor CCD chip.