Don't we need higher interest rates to encourage savings, to enable investment?
Raising interest rates could cause havoc in the financial industry. In addition to all the big banks, governments around the world would likely go bankrupt. Also, all the big insurance companies would be in a terrible position in that case. If interest rates would rise too high, that could cause the biggest recession in the history of the US.
Now I believe that's what the economy needs. A recession is the painful reallocation of misallocated capital. Only after those resources are finally put to more productive and profitable use can the economy ever recover and start to really grow again. But let's not kid ourselves, the transition would probably be very, very painful given what a huge mess we're in.
The Fed doesn't look at it that way. They want to maximize total output measured in GDP as well as achieve "price stability" (they inofficially follow a Taylor rule). The prospect of a deflationary recession is a nightmare to them. They focus on aggregated numbers and ignore the micro economic foundations of individual behaviour. And so far, they are doing their job. GDP is growing and the general price level is relatively stable. The fact that those two numbers are completely irrelevant is a whole different issue. From their point of view there is absolutely no reason to raise interest rates at this moment. I'd say it's more likely that they come up with some way to create negative nominal interest rates than that they would raise rates above 4-5%.