Most gravel tires are 38-40mm because that's what easily fit in "cross" bikes from 3-4 years ago that served as the base for gravel bikes. 40mm is also right at the limit before technical problems start cropping up at the bottom bracket juncture if you want to have non-mtb geometry and a 68mm bb shell + other stuff.
I went from 32->35->40 within the first 3 gravel events I did as it was obvious the terrain and climate down here in the southeast wasn't going to favor the narrower sizes. 40mm has served me well for the past four years but late last year I went and upgraded to 45mm. I haven't noticed much difference on pavement but I can pull out a little bit more performance on anything more technical than mild gravel compared to 40s. The extra little bit of speed down hill and cornering ability can make a huge difference over a 3-4 hour event. IMO it outweighs the slight weight, aero and paved rolling resistance penalties.