This.
Few points to note:
- Integrated sti shifter is more expensive than thumb shifters.
- MTB folks will buy a bike to upgrade, so having microshift 8 speed is not end of theworld. This means manufacturers aren't scared to mix and match.
- Brakes are not integrated so you get better brakes for less. For example, you can get tektro hydro brakes that will do the job, have to swap the whole thing for integrated shifters.
- Contrary to what we think, a carbon fork is probably more expensive than a lower end fork for oem (we get reamed in retail sales)
- The frame quality is probably a little better, needs to be lighter and the welds seem nicer. Most road frames will also be hydroformed (a lot of hard tails are actually not), you can generally tell by the shape.
- Frames are more standardised. A lot of entry level mtbs still running qr and IS mounts, then the higher spec ones will be completely new frames with thru axles etc. Gravel bikes tend to streamline their frame across the range so it may be thru axle to start with. Then the brakes will be flat mount, which is new and new = more expensive (new moulds etc.)
- Road bikes probably cost more to build. Just my expereince, for soem reason they take longer.