Because those stacks (Ruby/Django) come with automatic admin sections that let you build quite a bit quickly.
But now instead of exposing it as html pages, you expose your django/ruby models using an api like graphql or rest. So then you can use all new fancy js frameworks like react or vue.js.
Ruby and Django developers are as around as much as PHP developers.
PHP is not my go-to tool, but - you can do the same with Laravel (or a number of other packages). Also, my rough impression is that number of PHP devs outweighs Rails / Django at least 2 to 1.
But now instead of exposing it as html pages, you expose your django/ruby models using an api like graphql or rest. So then you can use all new fancy js frameworks like react or vue.js.
Ruby and Django developers are as around as much as PHP developers.