Their salaries are actually still fairly low. An engineer in SF can easily make more than $150k base per year.

Venture backed companies should pay their employees well though. If you're worried about food / shelter or constantly wondering if a shift in jobs would give you a disproportionate gain in quality of life then you're not focused on the startup. The biggest advantage of raising a lot of money is so that you can pay a lot of money and not have your team worrying about "I'd have loved to do that trip to Paris, but can't afford it. If I quit for Google, then I'd be good to go.

That said, I'm a believer that founders shouldn't be the highest paid employees.

Frequently, and I'm not saying it's Buffer's case, it's to offset the fact that founders have gone, possibly heavily, into debt over years of not paying themselves a salary.

Why wouldn't founders take such salaries, which are not outrageous?

Saying that founders shouldn't take a fair salary - comparable to what they might get elsewhere - puts them in a untenably risky position that would limit the pool of founders to the independently wealthy and the foolish.

That limitation would not be to the benefit of the backers.

Because they can. They've only raised a small angel round, so have no investors telling them what their salaries need to be.

Compare what $158K is in terms of adjustment in other cities. It is not very much money for a skilled worker.