For one thing, there aren't enough inspectors to check out workplaces on a regular—hell, even on an occasional—basis.
Bar chart of years needed for OSHA to inspect all jobsites, ranging from 30 years for Oregon to 288 years for South Dakota.

For another thing, the penalties for violations are low enough to be seen by unscrupulous or careless business owners as a cost of doing business. A serious violation of the Occupational Safety and Health Act drew an average penalty of $2,156 in 2012, and that's for states where the federal government is doing the enforcing. States that have their own OSHA plans give an average $974 penalty for a serious violation. Penalties are higher but still totally inadequate even when a worker dies:
For FY 2012, the median initial total penalty in fatality cases investigated by federal OSHA was $6,625 ...
Bad, right? Just $6,625 when someone died? It gets worse: After settlement, the median penalty was down to $5,175. And again, that's when the federal government is in charge. OSHA state plans have an initial median penalty of a paltry $4,900, which is knocked down to $4,200 by settlements. Criminal penalties? Forget about it:
They are limited to cases in which a willful violation results in a worker death and are misdemeanors. Since 1970, only 84 cases have been prosecuted, with defendants serving a total of 89 months in jail. During this time there were more than 390,000 worker deaths. By comparison, in FY 2012 there were 320 criminal enforcement cases initiated under federal environmental laws and 231 defendants charged, resulting in 79 years of jail time and $44 million in penalties—more cases, fines and jail time in one year than during OSHA’s entire history.
It's a slight overstatement to say that the government is basically giving employers free rein to kill workers. But it's not the outrageous hyperbole it should be in any kind of civilized society. Here's what some of those deaths look like. Grain smothering killed 26 in 2010:
“It created kind of a quicksand effect,” Piper said. “So we worked around it and we were aware of it, and after a while … Wyatt ended up getting caught up in it and started screaming for help. Me and Alex went in after him, and we each grabbed one side of him under his armpits and started dragging him out, and got pretty close to the edge of the quicksand and then we started sinking in with him.” [...]
“And it was just me and Alex standing there up to our chests completely, just trapped in the corn,” Piper said. “And Wyatt was underneath. I was hopeful that he was still alive, but at this point I’m pretty sure that he suffocated pretty quickly. The pressure underneath the corn was just too great.” [...] The corn kept flowing around Piper and Pacas. “After a little bit [Pacas’s] hand was sticking up above the grain and I could just see his scalp, and his hand stopped moving,” Piper said. “And the corn was up to my chin at that point. And it was slowly trickling down … and I was about to be covered, too.”
These deaths, like so many others on the job, are horrific and preventable, yet employers tell workers to take the risks, and don't suffer real consequences for the deaths.