When you think of it, allot of resources go into cockpit view but as the OP says not many use it. I think the option is nice to have but I do not use it when I race seriously but do sometimes when messing around. It is a good question. Having a Cockpit for the sake of it or because competitors have it is daft and a waste of good time and effort which could be better spent on something else.
Its good but shouold not be high on the list of priorities imo.
A lot of resources go into modelling exteriors too, and how many people use chase cam? If we disregard things like Photo-Mode and customization and assume for now most people use bumpercam, why waste time modelling cars at all when a simple photo of the real car in the garage screen would do and they only have to spend time dialing in the specs for the cars physics?
Cockpit view for me is one of the biggest, if not the biggest, additions to the GT-series since GT4, it completely changed the whole gameplay experience for me.
I only switch to other views when for example racing closed Standard cars casually as I can't stand the black cut out solution, or for example recently in GT-Academy I switched to bumpercam to extract the last hundreds of seconds and purely focussed on that purpose alone.
Other than that, I use cockpit view, also in online races as the enjoyment it provides me outweighs the potential loss of a few seconds (if it's that at all most times).
Before I joined this forum I assumed almost everyone would automatically love and use it instantly like I did, how wrong I turned out to be...
Different strokes and all, or old habits dying hard perhaps, but it's a given now and shouldn't just remain a high priority but part of the standard procedure.
Not saying it's perfect though, there should be an option to position the camera in the driver seat of each individual car given that they modelled the entire interior anyway.