It seems like a no-brainer...

No idea why there isn't one, perhaps a 2-4 part perk, 25% and 50% duration, and maybe all the way up to 100% increased duration.

Seems like a missed opportunity.

there would be less reason to create more potions if this was the case

Some ingredients can increase durations to 300 seconds (giant toe comes to mind) but yes, I would have loved a duration perk. Much more useful than green thumb and experimenter.