Because such gearing is IMHO useless on a gravel bike meant for mere mortals. I’m a relatively fit rider (last season’s peak was FTP = 342 W, 4.7 W/kg), and on an aero road bike even with fair winds and on a false flat, I rarely go much faster than 50 km/h, more like 37–40 km/h. I’m then in 42:14 to 42:12. I rarely use 42:11 and then use 42:10 much more often as an overdrive gear when my handlebars point downhill. I thought about gear ratios a lot before going 1x, and I paid close attention to my preferred gearing on the flats. The SRAM eTap app confirmed this numerically. The only thing I should add is that I am a bit spinnier than average, I like to do 95–103 rpm on the flats. I spin out at about 120 rpm, so with my setup I can reach up to 65 km/h while pedaling — faster than all speed limits. If I want to go faster still, it is usually better to tuck in and minimize drag. Plus, I have a wife and kids, and have no plans to become an organ donor anytime soon.
On a gravel bike with slower tires (as they have a profile and are wider) I don’t see any reason to get a larger chain ring. In fact, I’d probably go for a 38-tooth chain ring and think about a 10–36 cassette. 38:10 = 3.80 = 42:11 = 50:13 is plenty if you ride gravel. Yes, you might spin out at 46 km/h at a conservative 90 rpm, but how often are you going faster when you aren’t going downhill. Even when going downhill, how much faster to you want to be?
If you want to add gear ratios, I’d almost always advise people to add more climbing gears. So if you think 38/36 = 1.06 is too hard for some hills you are riding, that’s a good reason to get a bigger cassette. 38:44 = 0.86 is about 22 % easier than 1.06 = 38:36. Put another way, rather than grinding away at 60 rpm, you would do a more knee friendly 73 rpm with the larger cassette.
300 W is decent, so don’t sell yourself short. If you are unsure, pay attention to the gears you are riding. I have a hard time imagining you — or anyone else for that matter — are doing anywhere close to 45 km/h on gravel on a regular basis. Yes, perhaps you will spin out on the downhill, but think about it practically: how fast do you want to get and how much risk do you want to take?
The only other reason to opt for larger chain rings is efficiency, but also here I would say that this doesn’t matter for regular riders. Wearing tight-fitting clothes and learning to be in an aero position for longer will be infinitely bigger factors for us mere mortals.
PS One thing to add: I would stay away from Shimano. Their line-up doesn’t offer a lot of gearing options for gravel and you are forced to make weird choices. SRAM, Campagnolo and Rotor are much, much better in that respect. Especially SRAM can serve your needs 1x or 2x, and it is just up to you what you pick, they are agnostic about that. All of their choices are excellent, and it is really just up to you what you want.
Here is what I would go for if you went with SRAM:
- Red 1x crank with a 38-tooth chain ring
- Quarq DZero power meter
- SRAM Rival eTap Explore rear derailleur (which can also accept the 10–36 cassette)
- Rival eTap everything else
This combo is relatively affordable, yet you save a lot of weight with the Red crank. I think @WindWarrior has a setup like this on his road bike. (I wanted a Red crank as well, but couldn’t as it wasn’t available for another 2–3 months on top of the 4 months I had to wait for my bike.)
Even if you are wrong, you can still put on a different chain ring and/or cassette later.
With Campy, I’d go for the same gear ratios: 38-tooth chain ring and 10–44 cassette.