The cost savings can be a big factor, but that factor is not important to a lot of brewers.
I myself like the considerable savings over extract brewing, since even at the $35-40 I usually have to pay for a sack of grain these days (twice what I was paying only 15 years ago), that 50 or 55 lbs of grain still translates to anywhere from five to ten 5 gallon batches depending on the type of beer. The math is pretty compelling. And you don't even need to spend a fortune on equipment to go all grain.
So for me it's partly about the significant cost savings (especially these days) but in the end, I just enjoy the process.
Devoting 6 hours to a brew is not that big of a deal especially since I can typically do other work if I need to during the mash, the boil, etc.
But the other big draw for me is that I like the much better control I can get over the results as opposed to extracts. My first 15 years of brewing was pretty much all from extracts and I was happy with the results, but the last 25 years have been pretty much exclusively all grain, and despite the simplicity of extract brewing and the vastly improved ingredients available, I really don't think I could ever go back to extracts or even to regularly buying commercially made beer.