[...] about 400 job seekers inquired about thousands of job opportunities Tuesday inside a Market Street hotel a smartphone throw away from Twitter headquarters. Many of the nine employers at the fair, organized by HIREvents, said it's hard to hire in a city where the unemployment rate is 4.4 percent, a six-figure income doesn't guarantee you can buy a house and a company doesn't score cool points without stock options or an in-house barista. Recruiters seeking bus drivers, building managers, plumbers, regulatory analysts, and heating and cooling system engineers struggled to find folks willing to take a middle-wage job in a high-rent city. The problem isn't that there's a lack of workers, "it's that (employers) are not willing to pay them at the rate they want to be paid," said Sylvia Allegretto, an economist at the Center on Wage and Employment Dynamics at UC Berkeley. According to Allegretto, low- and middle-income job seekers - many of whom can't afford to live in the city - will face a difficult question: "Is it worth it for me to commute into San Francisco for this job?"