While both take starting position of the fielder into account, things like arm strength and batter run speed matter much more for IFers than they do for OFers. Just getting to a grounder means a lot less for an IFer than getting to a fly ball means to an OFer.
Unless I'm mistaken (I haven't kept up on IF defensive metrics as much as I should), IF defensive plays would have to be rated based on IFer starting position, batted ball speed, batted ball position, throw accuracy, and speed of the runner. I don't think they are that sophisticated yet, but there's really no reason they couldn't be since all that data should be available from the radar tracking system (throw accuracy to 1st base is captured by trackman? No idea...).
On the other hand, OF defense is measured by starting position, hang time, and end position. Trivial to compute and compare against other similar fly balls over the large sample set, and is why OF defensive statcast metrics came out almost immediately. Remember route efficiency?
Definitely something worth looking into some more.