I've got a hybrid bike with an 8 speed in-hub gear. I chose it because I didn't want to worry about the chain coming off or adjusting the derailleurs. It's been pretty bullet proof and given me no issues. The downside is, of course, that I only have 8 gears.
I was thinking why don't manufacturers put another in-hub gearing system in the crank? Even with 2 speeds at the crank that would double the range of gears.
Any reason why no ones done this?
Its unnecessary from a gearing range perspective and adds extra complexity, I suppose. Number of gears, number of distinct gears and gear range are all different things. Having 63 gears over the same 5.26x range that a Rohloff gives isn't much of an advantage unless you actually find the Rohloff gears are spaced too far apart (very few people do). You get complex shifting for little benefit.
People have made gearboxes in the bottom bracket and crankset (e.g. Pinion, Schlumpf, among others). You could put these with a hub gear or a derailleur system, if you wanted to (in fact, the Schlumpf link before markets the Schlumpf as a partner for the Rohloff internal gear hub), but you're adding another level of complexity to your drivetrain which gives you one more point of failure. Add that to the fact that you can have CVT's or 14 speeds in an internal gear hub (IGH), and you have more unique gear combinations than you necessarily need to begin with. In your proposal, you would probably add a bunch of redundant combinations. And I don't think theres a market for it.
That being said, if you want to go crazy, you could try to beat Sheldon Brown's 63 speed, which combines an IGH with a multi-speed cassette and a front derailleur (*). But with proper component selection, these days, you can get more gears than you could possibly know what to do with for IGH drivetrains (using a single chainring, but you could use a front derailleur if you wanted to, with a suitable chain tensioner) or derailleur drivetrains (using 1 or 2 or 3 or 4 chainrings; there are extenders to do more than 3 chainrings on a triple ).
(edit) The issue is torque limits on hubs. Rohloff publish theirs, chainring:sprocket ratio of 2.5 for heavy riders and 1.9 for most people. Shimano and SRAM don't, but have been known to refuse to service over-torqued hubs. So you can put a Schlupf Speed-Drive on on Rohloff to get about 4 more gears at the high end (Speed drive 1.6x, Rohloff spacing is 13.5%, log(1.6)/log(1.135)=3.7).
You could combine an 18 speed Pinion with a 14 speed Rohloff to get 252 gears over a 33.4x range. But the Rohloff ratio limit means that with a small 406 wheel you'd have a bottom gear of about 0.8m.... and a top gear of 26.7m taking you to 144kph at 90rpm. That's the current world speed record (2017).
(*) On the Bicycles.SE chat, I worked out a realizable drivetrain using off the shelf IGH's and derailleurs and what not that had way more than 63 speeds. Unfortunately, I don't remember what the components were. But, if you really want to, you could buy the parts and put them together to have your proposal work.
(edit) Perhaps a quad chainring with 12 speed cassette and SRAM 3x9 hub, modified to fit the 12 speed cassette.
