"I don't know why full stack webdevs are paid so poorly."

- The barrier to entry into webdev is low. You just need a laptop.

- An awful lot of websites seem to have been thrown together by morons; shipped; and never improved.

- For most sites, performance isn't an issue - strong tuning skills for PHP, SQL and Javascript are not an issue.

- A "full-stack webdev" generally doesn't do the full stack - few of my webdev colleagues have been interested in networks, for example.

TBH I don't know. In my last position I was paid £45K; I was the most versatile and experienced developer in the company (12 people). The bosses constantly complained that there was a dire shortage of talent to recruit. They recruited quite a few overseas visitors.