Total comp, I assume, includes presumed vested stock payout. Even as a FE developer (sr level) these numbers aren't unfathomable when they include stocks.
That's nice of you to say, but I think a few things play into it:
1) I've found most FE devs are not classically trained software engineers (most don't have CS degrees)
2) There's still a stigma that all FE is is writing HTML, CSS and JS (when, in fact, there's now entire build management layers, and the frontend stack is as complicated and nuanced as that of any OS' SDK). This is a holdover idea that is starting to diminish.
3) Because of 1, the market likely has more FE developers available to it. I'd be curious to see what your typical self-taught Java, C#, Python, Ruby or DBA makes.
4) I've also noticed a trend of BE engineers showing interest in front-end work, given its new challenges and such, further saturating the market. This is completely anecdotal though.